A vocabulary was the collection of words which made up a language. A language's vocabulary was often collected in a dictionary.
When Hoshi Sato tried to communicate in the Axanar language in 2151, she told Jonathan Archer she'd be lucky if she got half the vocabulary right. (ENT: "Fight or Flight")
When Vanna and Captain Kirk reached an agreement over zenite in 2269, Plasus angrily commented there was no such word as "agreed" in the Troglyte vocabulary. (TOS: "The Cloud Minders")
When Jean-Luc Picard asked Phillipa Louvois for help in 2365 in protecting Data, she initially rebuffed him, which made Picard claim that "trust" just wasn't in her vocabulary. (TNG: "The Measure Of A Man")
In 2368, Data compared the impossibility of understanding the Tamarian language without specific knowledge about the images used in it, to learning a language by knowing the grammar but not the vocabulary. (TNG: "Darmok")
In 2369, when Guinan asked Ro Laren how fun she had been as a kid, Ro replied that "fun" hadn't been in her vocabulary seeing she lived in a refugee camp. (TNG: "Rascals")
The same year, a diagram of the Bajoran wormhole, called "A Tunnel in the Sky" contained the following list of vocabulary words for Keiko O'Brien's students to learn: superluminal, space warp, wormhole, singularity, Hawking, Stephen, event horizon, Schwarzschild, Einstein-Rosen, Kerr Newman, negative space, millicochrane, warp factor, positive CTL, negative CTL, Kerr object, Godel Universe, sync shift, and time dilation. (DS9: "In the Hands of the Prophets")
When Seven of Nine didn't apologize to B'Elanna Torres for unauthorized repairs in 2374, Torres asked her if "sorry" wasn't in the Borg vocabulary. (VOY: "Scientific Method")
Later that year, The Doctor tried to teach Seven social skills with exercises he had created for himself in earlier years. When Seven complained those were for medical purposes and not suited for her specific role aboard USS Voyager, The Doctor replied the vocabulary of the exercises could be tailored to her specific needs later on. (VOY: "Prey")
In a deleted scene from "The Expanse", a book from the Starfleet Language Conference appeared in Sato's quarters with Andorian and Vulcan vocabulary.