Here's why:
* "Vocabulary" is a noun that refers to a collection of words.
* "Some" is a determiner that indicates a small, indefinite quantity.
* "Is" is a singular verb.
Since vocabulary is a collection (plural), you need a plural verb.
Here are some correct ways to write the sentence:
* Some vocabulary is useful. (This is correct if "vocabulary" is being used as a collective noun, referring to a specific set of words.)
* Some words are useful. (This is more precise and avoids ambiguity.)
Let me know if you have more grammar questions!