I am sorry, this might seem like a very stupid question for some of you, but as a non-native English speaker, I am having problems understanding what is vellum.
My searches show that vellum is some kind of paper made from animal skin?
Considering what vellum is used for, in the lore of FL, it seems to me that it has some other meaning? Could someone enlighten me please?
Sure. Vellum in the real word is exactly like you described it. In FL though, it's essentially the condensed essence of chaos. Usually (as I've RPed it) it takes the shape of a silvery fluid. All demons have some, sort of like blood but not as much.
Only powerful demons can conjure and command vellum, as it's Chaos made physical.
I hope that answers your questions. If you have any more feel free to ask.
Thanks!
In mythology, you'll also find that in Jason and the Argonauts, the Golden Fleece was made of vellum and in the lore of alchemy, vellum is the paper you describe, so alchemists believed that the Golden Fleece wasn't something worn, but rather a book of alchemy or a page of alchemy that held a great alchemical secret. Given that alchemy is the medieval version of chemistry (some would say lesser art, but given the relationship between science fiction and science, it's not too hard to see why people expect as much from science as they did alchemy), there could be some interesting theories on how the word "vellum" relates to the vellum of FL, which is what Dear Volg described. 
Vellum is mentioned in alot of the old demon help files as well. Most of my demon wanna-bes have been in search of vellum in some fashion.
