How about using symphony (sounds like "צמחוני") as a temporary memory hook? Make up some silly picture to visualize, maybe giant vegetables playing the instruments in the symphony orchestra, say "go to the symphony, be vegetarian /tsim-cho-ni/". Hey, I will definitely remember this word now, as well as the related word for "plant"!
In Semitic languages, the distinction between nouns and adjectives is pretty loose. Adjectives can stand on their own, they receive the definite article, they decline the same way nouns do, and many of the names of everyday objects are just adjectives formed from verbs. So I'd say it works the same way as in English: אני הצמחוני = I am the vegetarian But this applies to pretty much all adjectives in Hebrew: I am the one who sees = אני הרואה