vellum vs parchment

vellum and parchment both are nouns.

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

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

Other hypernyms of vellum include lambskin, sheepskin.

vellum (noun)	parchment (noun)
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	a superior paper resembling sheepskin