[Supperman]

Thank you very much, susan53, for your detailed explanation and the information about Google. It became clearer now.

I just searched;
"English vocabulary is XX"
XX.........numbers hit
limited....26700
poor........2490
small.........256
lacking......239
short..........49

good......1870
rich....... .416
excellent..254
large........211
great.......171
adequate.....2

According to your advice, I wrote example sentences.

I've learned three languages; Japanese, English and German in my life. So I have three vocabularies. My Japanese vocabulary is, I would like to believe, adequate, because it is my native tongue. My English vocabulary is limited but I can communicate to some extent for international communication by using English. Whereas, my German vocabulary is so limited that I can only speak some greeting expressions.

Is my understanding about the usage of plural of "vocabulary" correct in this context?

Thank you.