How does a vampire level up?

Undeath is about exchanging a semblance of life for a semblance of power. That's a basic premise of this game.

How do I make that work mechanically?

By chopping apart the "roll on the number" mechanic of Lasers & Feelings, turning it into a "roll on or under" thing where the more undead you get the easier it is to get more undead ... until at the peak of your power you become only hunger and leave play.

Everyone has an expiration date.