Does the word "vocabulary" have plural?

Yes you can. The plural of "vocabulary" according to the Oxford Advanced Learner's Dictionary is "vocabularies". I also checked it on another, software dictionary called WordWeb.

But I'm not sure if the plural form should be used in that sentence as well. In my opinion, if you're talking about just one language, you should use "vocabulary" instead of "vocabularies". The latter word should only be used in case you're referring to multiple languages...

The vocabulary [ singular ] of one language - singular - he is learning German vocabulary, he is learning the vocabulary of Polish.
He is learning the vocabulary that is used in everyday conversation.

vocabularies - plural - mean ther word stock of more than one language. He is learning the vocabularies of Polish and Urdu at the same time.

What Is Vocabulary

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singular "atheist" instead of a plural is the word used as an adjective, like "heathen" is a pejorative adjective. They're dumb in them there parts,lol

hard task. browse at a search engine. just that can help!

Yes, vocabularies is the plural form.

vo·cab·u·lar·y

vo·cab·u·lar·y [vō kábbyə lèrree, və kábbyə lèrree]
(plural vo·cab·u·lar·ies)