That depends. Didn’t the medieval scribes sometimes scrape off older texts already on the vellum , and write new manuscripts on that same old vellum because vellum itself was so scarce?
No, they were a hidebound lot.
Okay, I have no idea if they did or didn't.
Use of old vellums. These reused manuscripts were called “palimpsests”, and I would expect that the majority would be reused within the same monestary, therefore having the value suggested in the article. Of course, if they were decrees or books that were sent elsewhere, then there might be a distance problem.