Thank you for playing! We have noticed the Vampire fight doesn't always work, and actually have a big polishing update very Very close! Please swing by the next couple weeks, hoping to have it uploaded soon.
As for the Vampire Weakness, it's not easy to notice, but when a Cards power is double or more of the Enemies Armor Class, the attack hits true, regardless of weaknesses and resistance. So for the Vampire, when he's in his human form, you need really big value cards.