I think that foreign language teachers need to know all (or very nearly all) the grammar in the language. Vocabulary is something different, though. I think that foreign language teachers should know high frequency words and the 200 most commonly used verbs, nouns, adjectives, etc. It's darn near impossible to know the meaning of every word in a language, even one's native language, so foreign language teachers just need to do the best they can in that respect. At a minimum, they should know all the vocabulary words in their textbook and/or the passage/poem/prose being studied. That's my opinion, anyway.