As for the prom dress, you could just plan an outfit. As far as I know, there are no specific “prom dress” outfits, so let her wear whatever she wants to. You could definitely build a store that sells prom dresses, and perhaps tuxedos and other formal wear as well. She could attempt to find one at the thrift store. One of my sims found her prom dress at ThrifTea.
As for the vampire mentor… I don’t have that pack. But there’s always Vlad. Not sure he’d be the best mentor though. Depends on if she wants to be a “nice” vampire or a… not so nice vampire, I guess.