vellum vs lambskin

vellum and lambskin both are nouns.

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

As nouns, lambskin is a hypernym of vellum; that is, lambskin 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
lambskin: skin of a sheep or goat prepared for writing on

Other hypernyms of vellum include parchment, sheepskin.

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