A pack of hyenas will eventually get full and lose interest in the decanter of endless brains (which may be the Best Item Ever, btw

), because theirs is a natural hunger.
The question is whether the ghouls will ever get full with an endless brain supply? Or alternately, will they ever die if they never eat?
If they feed because they need sustenance (ie, metabolize), then I'd rule that the ring will work. If they feed because they have an unnatural/spiritual/infernal desire to eat brains, then I'd say the ring of sustenance won't work, because they don't need sustenance. I can see it going both ways, depending upon what it means to be a ghoul. Same for the vampires.
IOW, it's highly game specific.
Aside: since 3e vampires gain temporary hp by feeding on blood, do they gain temporary hp with a ring of sustenance? I'm inclined to say "of course not". But if vampires
metabolize blood, then I'm less certain....