Jeanie, do you have the slightest imagination how expensive glass was in those times? Where could those remote natives of Old Britain raise the money to buy it for scratching their leathery hide?
Vitrum (vitro is the ablative) may mean glass also, but when used with colouring you are on the safe side if you presume that it means woad. In all his commentaries Caesar uses the word vitrum only once, viz. in the chapter I gave above, and there it clearly means woad.
Inficere means "to put on, in". It fits here well for putting on a dye. Other suggestions remind me of my early theological and juridical studies, where words and meanings were bended to the extremes just to have a support for thoughts which couldn't be proved otherwise.
Caesar was an old sweat in the army and reported mostly correct what he saw, except where he relayed on hearsay (comm. 6, 27: the elks in Germany). I think you should believe what he wrote down.

Wilfried