It would seem Vampires would be the last to have anything to fear from such creatures of darkness, but this is not necessarily so. More than one Kindred has met her end in conflict with a Demon, and others have dealt with them only to come out the worse for the bargain. Some Cainites laughingly tell of having sold their nonexistent soul to demons, though others shudder at the thought.
Some demons have evidenced a particular interest in the Undead, though the reason for this remains unclear. Whatever their purpose, however, they are invariably friendly to Kindred unless they feel they are crossed, in which case their rage is truly horrible to behold.
(SH pg 101)

Demons themselves rarely manifest upon the Earth; instead, their spirits inhabit lesser forms, mortal beings with supernatural powers, which are here called the infernal hosts. Occasionally possessed, often created from scratch in an imperfect mockery of the Lord’s own creations, host creatures exist a short time before decaying. Unlike a demon’s true form, these creatures can be hurt or killed.
Occasionally, some ritual or rift between Hell and the Earth allows a demon to create a material aspect of its true form. These manifestations are very rare and exceedingly powerful. No mortal, and few immortals, could stand and fight one on its own terms. Those who would bind or traffick with the Infernal must resort to elaborate magics or divine aid. The latter requires more faith than most Cainites can muster, while the former leaves a magician’s soul at risk. Even if a demon’s material form is slain, the entity itself is only banished back to the Inferno. This banishment may hurt, but will not prevent the Fallen One from coming back at another time to avenge itself. This can be especially problematic for a vampire; after all, you can accumulate a lot of enemies in a century, and not all of them reckon their lives in mortal years....
(VDA 266)
In simpler words, a demon's host (material) form can be killed, but the demon can NOT be. He will only return in another host form and nail your ass.

Demonic Powers
Healing
Can heal one Health level per turn without effort. Really powerful manifestations can heal two, three and even four per turn.

Fortitude
Devils, in material form, are outrageously tough, and soak aggravated damage from all sources except the holy magick. In V:TM, this is up to 5 in Fortitude.

Domination
With a glance, an Infernal creature can cow most mortal beings. In V:TM, demons have several dots in Presence and Dominate.

Fire
Although many of them do not use it, all demons can surround themselves with flames, and hurl fire at their enemies (5-10 dice of aggravated damage, depending on demon's rank).

Immortality
No demon or evil spirit can be truly killed--at least not by anything that walks the earth. The creature's material or spiritual form can be destroyed, but that simply banishes the Infernal soul back to its Realm for a time...after which time it will be ready for revenge.

Immunity to Fire
Accustomed as they are to the fires of Hell, demons cannot be hurt by any form of mortal flame (magical or otherwise).

Terror
Demons can inspire paralyzing fear in most mortals. In V:TM this comes through lower levels of Daimoinon.

Most demons wield the following powers:
o Healing: Demons and their host creatures can heal one Health Level per turn without effort. Really powerful manifestations can heal two or even three Levels per turn.
o Fortitude: The Infernal are notoriously hard to hurt. Assume that any demon-form has Fortitude ranging from 1 to 5.
o Domination: Those who meet infernal manifestations know it; even the lowliest demons have at least two dots of Presence, and many have comparable levels of Dominate as well. Powerful demons and hosts can cow an entire room of mortals with a glance.
o Daimoinon: An innate demonic power. Lords can go as high as 8 with this Discipline (which allows them to summon others of their kind or curse a large area), and dukes and their betters can attain any level.
o Immunity to fire: Flames are demons' natural element; no fire-based attack can harm them except for their own infernal flames (which hurt but inflict no damage).
(VDA 265>

Weaknesses
Although they carry hellbound powers, infernal beings are subject to the power of God's creation. While few Cainites could (or would) beseech the Almighty for help, the following things can harm or repel a demonic manifestation:
True Faith:
Faith has double its normal effect on infernal powers. A character with Faith can inflict one die of aggravated damage for each point of Faith she possesses. Faith can come from beliefs other than Christianity; Islam and Judaism have long histories of divine favor, and even good pagans can resist the lure of darkness.

Pride and greed:
Demons are a crafty lot--sometimes too clever for their own good. Because they consider themselves superior to humanity, some can be tricked into leaving or performing short services. This is exceedingly dangerous, however; many tempters will play dumb, let a character believe she has the best end of a deal, and then spring the trap.

The barriers:
God does not allow demons free reign on Earth; they must be summoned, bound or brought into the mortal world through human evil or stupidity. Once they're there, most demons choose to stay awhile. Some magical charms might raise a similar barrier.
VDA266

Power
Demons are always willing to teach their dupes new powers. Almost all infernalists are taught Dark Thaumaturgy by demons, and many vampires seek out demons to learn Disciplines they cannot gain from their clan.
Dark Thaumaturgy (varies)
Characters can purchase Dark Thaumaturgy from their demonic mentors. Ratings in the paths of Dark Thaumaturgy can only increase one point at a time. For example, if a character has Video Nefas at Level Two, he can only increase it to Level Three with the next pact he makes. The character need only make a pact equal to the new level of Dark Thaumaturgy, but must immediately undergo a Supplicium upon learning the corrupt knowledge.

Disciplines (varies)
Demons can teach Disciplines to vampires. Each time a Discipline is increased through an infernal investment, it may be raised only one level. Raising Disciplines costs the character a pact equal to the new Discipline level +3. A demon can also teach the character a single power from a Discipline (for example, just Dread Gaze, instead of the Discipline of Presence). This costs the character a pact equal to the level of the specific power +1. In this way, mortals can receive these specific powers from demonic mentors. The special infernal Disciplines of Maleficia and Striga (as well as any other infernal Disciplines the Storyteller creates) can only be learned by dealing with demons.

Attributes and Abilities (varies)
Demons aid their minions to become more powerful. Attributes and Abilities can he raised only one level for each pact. Attributes require a pact equal to their new level +2 and Abilities their new level + 1 to raise through infernal means.

Life (one point per year of life)
Though inappropriate to vampires, demons offer mortals extended life in exchange for pacts. The mortal's life (in good health) will be extended for one year for each point of the pact. The demon can also reverse the aging process for the same cost. A demon will never grant unconditional immortality, nor will it guarantee anyone against harm. Similar pacts may also be made to cure diseases and heal wounds.

Servants (varies)
Minor demons may serve a character in exchange for pacts. Powerful infernalists may be attended by many followers. Most such servants are possessed mortals who are intensely loyal, and even willing to die for the infernalist. Such servants cost one point unless they are especially skilled or powerful. Bound demons may be granted to an infernalist for two points, and it is possible that a more powerful demon will consent to serve for a sufficiently large service.
(VDA COMPANION 177)

THE DEMONS
The true host of Hell is made up of countless demons. It is these beings who are bought into the world by infernalists and most readily take an interest in the affairs of mankind. Most of them are subservient to their masters in Hell, but there are some who seem worryingly independent. According to medieval scholars, six broad types of demons have access to the world.
They are called daemons to distinguish them from their more comprehensible kin. lnfernalists have little power over these beings; they may only be summoned using True Magick. Astrologers have said that a few of these star daemons have shown an interest in the growing study of mathematics and other natural sciences at the University of Paris.

Star Daemon
This being appears as a twisting ribbon of light and shadow. Shapes emerge from this formlessness, hinting at geometric patterns or even writing. Gazing at this being too long makes the viewer uncertain of her place in the universe, as she begins to sense the vast void that surrounds her. This daemon is angered by any attempt todispel it, using its Thaurmaturgy and Obtenebration against such an attacker.
According to translations of Chaldean and Egyptian magic, there exists a breed of greater star daemons who, like the wandering stars, intersect with the world only at specific times. The Sphinx and some Babylonian carvings are said to depict these beings. The powers of such inhuman intellects can only be guessed at.

Aerial demons
These demons swarm invisibly through the spirit world, seeking entrance to the material world in order to tempt or attack humans. They conspire with infernalists for the destruction of mankind.

Terrestrial demons
Terrestrial demons are the most dangerous sort, for they have been cast out of the spirit worlds and dwell on Earth among humanity. They are often confused with werewolves and fae, for they are adept at changing their forms and casting illusions. The weakest among them possess the bodies of animals, creating Hell-beasts, while the more powerful pose as human beings and work their corruption secretly. If a terrestrial demon is slain, it is truly dead rather than merely dispelled.

Aqueous demons
These demons dwell in rivers, lakes and the ocean depths. They delight in drowning humans, especially children, and use both guile and outright force to claim their due. The aqueous demons of the ocean are colossal monsters capable of crushing ships in their coils or tentacles.

Subterranean demons
Subterranean demons dwell deep within the earth and in caves and caverns. They attack those who disturb their domains (miners and treasure hunters especially), but are otherwise encountered by humankind only when summoned to the surface. They are slow and malicious, appearing as hybrids of armor, flesh and stone.

Bound demons
Least among infernal powers are bound demons. These demons are so pitiful that they can be summoned and commanded to work by mere human magicians. Bound demons are usually small and brutish, often appearing as overly large toads or other vermin, and serve either as familiars (for the length of the magician's life) or as guards set over places or treasures.
Familiars are tiny demons that are bound to a specific master. They generally appear as small animals (toads, cats, weasels and ravens are common) and serve as spies and gossips. Familiars need to be fed the blood of their master weekly or they will wither and die.
Custodes are minor beasts bound to guard a place and its treasures. They recognize only the one who summoned them and attempt to eat all other trespassers. They are dull-witted and stow, but immensely strong and persistent. Most of them appear as large spiders or giant toads covered in caustic slime. A few may have poisonous bites or other abitities. (VDA COMP 182-185)