@Simmerville
1. Would you say the pack is worth 50% off for mainly the world and some of the BB&CAS?
Yes, 100%. The world is cool, but the cas/bb is incredible and I would heartily recommend, especially on sale.
2. If I have no vampires living in any house, would there still be game generated NPC vamps?
I think so, but as I always leave the premades in I'm not sure.
3. Will the CAS face details be available only to vampire sims?
No, other occults can access them as well.
4. Can garlic be used for anything but keeping vampires away?
Yes - like with all plants you can make money from gardening it, and it also is used in most of the drinks and recipes that come with the pack.
5. Can vampire stop being a vampire, and if so, will s/he keep those perks earned?
Yes vampires are curable, no they will not keep their perks.
6. Do vampires live forever? If so, the family photo wall might be kinda limited as they hardly need to reproduce... Or does it vary, or can it be activated/deactivated like a perk?
Yes, vamps live forever - once they reach young adult they stop aging (but can still be aged up with the cake, like any other sim). As a compensator, they're pretty easy to kill.
7. Are any features restricted to the Forgotten Hollow world?
Not really - the plants from the pack exclusively spawn there, and the day/night cycle is different, but asides from that no.
Anything else I should know?
It's the best pack for the Sims 4, if you like occults at least. Vampires are super fun to play, and they have a weakness system that makes it so that there's actual fun challenge to the gameplay. And an update a few months ago made it so that you can turn off vampire attacks for certain households, which nullifies the main aspect of the pack that people didn't like.