mental lexicon vs vocabulary

mental lexicon and vocabulary both are nouns.

Word	Noun	Adjective	Verb	Adverb
mental lexicon	Yes	No	No	No
vocabulary	Yes	No	No	No

As nouns, mental lexicon and vocabulary are synonyms defined as:

Other synonyms of mental lexicon include lexicon.

mental lexicon (noun)	vocabulary (noun)
a language user's knowledge of words	a language user's knowledge of words
	the system of techniques or symbols serving as a means of expression (as in arts or crafts)
	a listing of the words used in some enterprise