[susan53]

vocabulary can be countable or uncountable:

1. With the meaning "some words" it's uncountable and singular : Today we're going to learn some vocabulary.
2. With the meaning "a person's total knowledge of the lexis of a language it's countable and singular : She's grammatically inaccurate but she has a good vocabulary.
3. It's countable and plural when used as a synonym for dictionaries, and especially in technical contexts such as computer programming.

You sometimes used it with meaning 1 and sometimes with meaning 2, so the grammar changes. For example :

She only has a small vocabulary / She doesn't have a very large vocabulary (= meaning 1, countable)
or
She knows a lot of vocabulary / She doesn't know much vocabulary ( = meaning 2 - uncountable)

You could also say :

Her vocabulary is poor/not very good /limited/adequate /good /excellent.

She has a basic level of vocabulary
Her vocabulary is at advanced level


She has/doesn't have) a wide (range of) vocabulary.
She has a restricted /limited range of vocabulary.
She lacks vocabulary.


However, it's difficult to give clear rules here about when you can use which adjective. It depends on things like :
- Are you using meaning 1 or 2?
- Is the sentence affirmative or negative?
- Is an adverb (quite, very etc) used to modify the adjective?
- Are you also using an expression like "range of" or "level of" ?

It's a complex area. if you use one of the expressions I've given above, exactly as I've included them, it will be OK. However, if you change the sentence from affirmative to negative, omit the adverb etc, it may no longer sound right.

One way to check is to type the phrase you want to use into Google and see if it comes up frequently. Use inverted commas to search for the complete phrase - eg "I have a large vocabulary" or "my vocabulary is limited"

[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.