I couldn't find "vasalier" in any French dictionary, regular and historical, online and mine, even in Old and Middle French dictionaries.
In Medieval France, a vassal was someone who had sworn an oath of allegiance to his liege lord and/or was "below" in the social hierarchy -the same way as in England I think. But you wouldn't say that vassal was the name of a trade.
I put the word on Google and apart from this thread there's "1890's Normandy Louis XV Vasalier" = mispelling for vassalier = Fr. "vaisselier" = piece of furniture which name derives from "vaisselle", there're two persons' names from Southern France so I searched a couple of Occitan dictionaries to no avail, I also came across a German forum and the word is related to "vassal" -Middle Ages concept.
The only interesting thing I found was when I searched the etymology of "vassalier" : I came across that "En Provence, on rencontre plusieurs noms qui semblent dériver de Vassus, serviteur: il s'agit de VASSEL, VASSELOT, VASSELON, et peut-être VASSALIER." = "In Provence, we come across several names that seem to derive from Vassus, servant: it can be VASSEL VASSELOT VASSELON, and maybe VASSALIER"

So I think that Noreen may have it right.