Been playing it a bunch. Vampire definitely feels far, far, far stronger than the knight (stress heal when full, maybe more loot due to food not spawning, and ability 3-shotting the boss if you save it until the end).
I would suggest buffing the knight so that a 2nd armor card gives 75%, or 100% of the value, and removing vampire's stress heal.
That said, the game's balance is in a pretty great spot right now (challenging when you start, fairly consistent to beat once you know what you're doing, low/no frustration towards RNG outside your control).