General synonyms:
* Lexicon
* Word stock
* Word hoard
* Word list
* Word knowledge
* Word power
* Verbal repertoire
* Language proficiency
More specific synonyms:
* Technical vocabulary: Jargon, terminology, specialized language
* Literary vocabulary: Diction, phrasing, literary language
* Informal vocabulary: Slang, colloquialisms, vernacular
The best synonym depends on what aspect of "vocabulary" you want to emphasize.