Majin Vegeta is plain and simply Vegeta with an M on his forehead that stands for Majin.He let Babidi take over his mind to get the power. But the thing is,Babidi couldn't control him.
Kai Gohan:This form of gohan is as powerful as a super saiya-jin 2,but gohan didn't have to be a super saiya-jin 2 to have the power,he could just be normal.Gohan trained with the Z sword and when he swung it so fast it broke and an old kai came out and gave him a kaioshin upgrade by doing a mysterious dance but did not finish.It is said that if the kai did finish Gohan would have been as powerful as a super saiya-jin 3 without transforming.