The Romans were famous for their glass, the term vitrum for it appears in other works that Caesar's.
Dictionaries, unless supported by dated entries (like the OED), are useless in determining when a meaning was in use. The appearance of "woad," or "dyestuff," as a definition of vitrum in abridged modern Latin dictionaries does not mean that the word had that sense in ancient Rome. Many additional definitions were added in medieval times.