lambskin vs vellum

lambskin and vellum both are nouns.

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

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

lambskin: 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

lambskin (noun)	vellum (noun)
the skin of a lamb with the wool still on	fine parchment prepared from the skin of a young animal e.g. a calf or lamb
skin of a sheep or goat prepared for writing on	a heavy creamy-colored paper resembling parchment