sheepskin vs vellum

sheepskin and vellum both are nouns.

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

As nouns, vellum is a hyponym of sheepskin; that is, vellum is a word with a more specific, narrower meaning than sheepskin:

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

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