vocabulary vs mental lexicon

vocabulary and mental lexicon both are nouns.

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

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

Other synonyms of vocabulary include lexicon.

vocabulary (noun)	mental lexicon (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