When you divide a word into its individual vowel sounds, that's syllabification. The syllabification of "vocabulary" looks like this: vo-cab-u-lar-y.
The word syllabification essentially means "the act of separating into syllables," and syllable goes back to the Greek syllabē, "that which is held together" or "several sounds taken together." Syllables are sounds held together by vowels, and the process of syllabification involves identifying those separate syllables, in speech or writing. Syllabification is a good way to learn a new word, whether you're trying to spell it or pronounce it correctly. Try it with syl-lab-if-i-ca-tion!