I am deciding which one to get first - I can get them for about the same price.
Cynders or Vampire mission pack?
Bit of an apples to oranges comparison. With Cynder or Vampire MP, you get 1 new enemy. Vampire Mission pack you get 1 extra tile, 4 missions and the cards focus on the vampire. Cynder does not focus on the Lava Men, but instead gives you a whole new world, with a lot more cards, tiles the whole shebang. If you have the Swamps of Terror Core set, the Hellbat and Hungry dead also get a different version specifically for Cynder so you can repurpose the miniatures for Cynder. If you don't, you get tokens for the monster variants. If you're happy with the Otherworld that comes with your Core set, then Vampires expand on the Core Set. But if you're itching for someone else to go, then Cynder is the best choice.