[split] Did the VM go straight from cerebellum to vellum?

[Rafal]

So, the question remains.  Are there surviving instances of Medieval manuscript books on vellum that are known to have been scribed by the author himself?

I have found it in 5 minutes, and it is actually quite late:
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Parchment · 490 pp. · 22.5 x 15 cm · Hochheim near Worms, Dominican Cloister of Maria Himmelskron · about 1474
Composite manuscript from the Dominican Cloister Maria Himmelskron in Hochheim near Worms, containing works by Johannes Meyer; according to a note of ownership in his own hand, it was written in 1474.

So probably there are a lot of them.
Or do you mean vellum is not the same as parchment but only the finest, most luxurious and expensive parchment?