Betty Jean Ward began providing Velma's voice in 1997 with the Scooby/Johnny Bravo crossover.
She continued as Velma with the making of The Scooby-Doo Project (1999), a parody of The Blair Witch Project.
B.J. is Velma in the episode of Harvey Birdman: Attorney at Law (Season 1, Episode 3) in which Shaggy and Scooby are put on trial for marijuana possession and resisting arrest.
She is Velma in Night of the Living Doo (2001 Halloween special), the movies Scooby-Doo on Zombie Island (1998), Scooby-Doo and the Witch's Ghost (1999), Scooby-Doo and the Alien Invaders (2000) and Scooby-Doo and the Cyber Chase (2001), as well as in various Scooby video games.
Behind The Voice Actors states that B.J. "Often wore a turtleneck sweater and glasses to Scooby-Doo recording sessions to help get into her character of Velma."1
B.J is Velma the Playboy Bunny because "Before she started doing voice work she was a Playboy Bunny."2
Information Sources:
1B.J. Ward. Behind The Voice Actors. Retrieved from http://www.behindthevoiceactors.com/BJ-Ward/
2B.J. Ward. IMDb. Retrieved from https://www.imdb.com/name/nm0911389/bio?ref_=nm_ov_bio_sm