You can get vellum easier than hemp paper???
In the US anything hemp is hard to find.
That includes hemp rope, hemp fabric, and hemp paper.
I have a friend that makes hemp kilts. He has to import the material from Canada. He also has had to argue with cops and customs that it isn't drug paraphernalia . . .
Sometimes, the US makes blanket laws that sound good on the surface, but also imply in many unintended and rather odd ways.
Though, modern vellum isn't always what vellum used to be. Real vellum is paper made from a sheet of very thin leather. It is near indestructable, often reused in ancient and medieval times. One document (forgot which one), it was discovered by new modern testing techniques, had been used many, many times. What they would do is to wash off the ink and reuse the vellum.
Modern vellum is a plastic-cotton paper, made to look and feel like vellum, but isn't actually vellum.
http://en.wikipedia.org/wiki/Vellum
(It is easy to read and, in general, fits with the facts I know.)
I'm not sure which form of vellum was meant in the early 1700s, though.
I assume it was actual vellum.
I don't know where o buy actual vellum . . .