I think the theory is that it is precisely because
vair was an obsolete word in French is why Perrault, the author of the most famous version of
Cinderella, confused it with
verre. But there does seem to be a lot of disagreement on whether or not he did this deliberately.
Cinderella is actually the oldest one of the "fairy tales" still popular in the Western world. The oldest version of it found so far is from
China in the 9th century A.D., so the basic tale is over a thousand years old.