I was watching a Dilla Documentary with Madlib hunting through Dilla's record collection, and he stopped himself from mentioning what a couple records got used for on Donuts, because he was pretty sure they weren't properly cleared. On Donuts. Huuuuge record. Uncleared samples. His Moog in the Smithsonian.
I think we can all agree that it's a catch-me-if-you-can game, as long as you are turning something, especially something dusty and forgotten, into something totally else entirely.
Snatching whole vocal lines from someone without payment or credit, probably not so good, in terms of liability. Dick move.