vellum vs sheepskin

vellum and sheepskin both are nouns.

Word	Noun	Adjective	Verb	Adverb
vellum	Yes	No	No	No
sheepskin	Yes	No	No	No

As nouns, sheepskin is a hypernym of vellum; that is, sheepskin is a word with a broader meaning than vellum:

vellum: fine parchment prepared from the skin of a young animal e.g. a calf or lamb
sheepskin: skin of a sheep or goat prepared for writing on

Other hypernyms of vellum include lambskin, parchment.

vellum (noun)	sheepskin (noun)
fine parchment prepared from the skin of a young animal e.g. a calf or lamb	a document certifying the successful completion of a course of study
a heavy creamy-colored paper resembling parchment	tanned skin of a sheep with the fleece left on; used for clothing
	skin of a sheep or goat prepared for writing on