Vampires is the best pack ever, as far as I'm concerned, and you really can't go wrong with it if you want something different. It's my favorite because it adds some much needed darkness to Sims 4 and everything in the pack is interesting and fun. It's very flexible and allows you to play a sort of "vegetarian" polite Vampire if you don't want them to feed off Sims and prefer them to be well behaved. On the other hand, you can really stir things up with more evil sorts and amp up the creep factor too, which I have a blast with. >:)
There's a lot of gothic Victorian style to both CAS and Build/Buy, as well as items to make a building look nicely run down. There's some cool gargoyle statues, spider webs (that have active spiders!) murky plants, dramatic wallpaper,...and in CAS there's a variety of Vampire features, fang types, glowing and creepy eyes and really cool outfits.
The Vampire powers are awesome and there's a great selection that allows you to customize your Vampire's strengths and balance it out with weaknesses. Vampires can also battle each other and Werewolves too, if you get that pack.
Also, Vlad is the best NPC in the game. I said what I said. ?