When I was discussing “untranslatable” German words and phrases with my students at MIIS Monterey, one of them suggested eingefleischter Vegetarier. That is indeed a tough one to reproduce! My best attempt is rabid vegetarian, since “rabid” alludes to the animalistic in the same way that “eingefleischt” does and introduces a similar (humorous) dissonance.

Can anyone propose a better solution?