I thought I knew the meaning of Vocabulum and Capitulum - but checking up in Cassell’s and Collins’ dictionaries I’m surprised that they don’t come up with the expected translations. Cassell’s: capitulum = (dim. of caput) little head; under caput = heading. I’d suggest sub-heading for capitulum.
Collins: no capitulum entry; under caput: chapter, main point. Vocabulum = name, designation, noun (but not: word). Cassells: name, appellation, a noun substantive. - Just saying. 
Why dont you get a reliable small dictionary such as the “Elementary Latin Dictionary” of CT Lewis? Amazon have plenty of hardback secondhand copies for £12 plus shipping.
I am not sure what you expected as a translation for “capitulum”. One can always use the Latin word study tool at Perseus to look things up in Lewis and Short. See here http://www.perseus.tufts.edu/hopper/morph?l=capitulum&la=la#lexicon.
As for “Vocabulum” Lewis and Short doesn’t gloss it as “word” neither for that matter does OLD.
Chapter? Thanks for the links to perseus. My post was slightly tongue-in-cheek.
This is what happens to people using Ørberg – those are the words he uses for 'chapter" and “vocabulary.” He actually borrows from later Latin with capitulum. From Smith and Hall:
chapter:
I. A division of a book, etc.:
- căpŭt, ĭtis, n.; esp. of the divisions or chapters of a law: from the first c. of the law to the last, a primo c. legis usque ad extremum, Cic.: of a book: Cels. (and later Latin, passim).
- căpĭtŭlum: Tert.: Just. Cod.
The OLD on vocabulum:
uocābulum ~ī neut. [uocō1 + -bulum]
1 A word used to designate a thing or idea, a term, name, etc. (incl. proper and common nouns).
To me that seems to fit. Cicero seems to come close to meaning what we mean by it:
an hoc dubium est quin neque verborum tanta copia sit non modo in nostra lingua, quae dicitur esse inops, sed ne in alia quidem ulla, res ut omnes suis certis ac propriis vocabulis nominentur, neque vero quicquam opus sit verbis, cum ea res cuius causa verba quaesita sint intellegatur?
Cic.Caec. 18.51.
Before Ørberg, I would simply have used caput and verbum.
Some of his other vocabulary choices surprise me as well, such as ostium rather than the more familiar ianua for door.
Are you saying: Abandon all hope! 
But seriously, I am beginning to realise why the apparent need for so many different translations of the same Latin texts. It’s a minefield for linguists.
The OLD on vocabulum:
uocābulum ~ī neut. [uocō1 + -bulum]
1 A word used to designate a thing or idea, a term, name, etc. (incl. proper and common nouns).
To me that seems to fit. Cicero seems to come close to meaning what we mean by it:
Context is everything.