Possibly my favourite pack, I highly recommend it! Vampires won’t autonomously turn your sims, but the game will spawn a few vampire npc’s to populate your world.
Occasionally, not often, but sometimes, vampires will turn up at your sim’s houses at night, but you can stop them entering by clicking the entrance doors to the building. I’ve never seen one actually try to bite my sim autonomously after years of playing, but they can do it, it’s just rare.
Oh as for the sales, they tend to happen every 2 months so you won’t have to wait long to get it at a reduced price if you have the patience.