[Snake]

That doesn't happen with the Zeran racial passive that reduces WIL, or Sanctuary. You 'can't' lose skills mid-battle.

If anything, the Vampire will end the battle with less WIL/SKI and lose his slots 'after', if they decide to spend Essence like a bunch of careless immortals. It's yet another good drawback for Silvermist spammers, and gives the feel of 'a vampire has been weakened', especially when fighting a bunch of enemy waves.

And if the Vampire manages to end the battle without full Essence, they're digging their own grave, really.