grunger (26a). Yes, I like the clue too, and I think the thematic element is nicely worked, it's only the missing anagram indicator that bothers me (either that or, as seems likely, a part of the definition is doing double-duty as def/anagrind).
Chambers' definition is 'interjection expressing Bacchic frenzy', I can't see how 'frenzied' can be removed from the definition to act as an anagrind (and without it there isn't an indicator that [a]EIOU becomes EUOI)
I can see what you're getting at re the order, but I'm not keen on giving the vowels an arbitrary order at the outset because a quintet should be an organization of parts - and that organization has always been AEIOU where vowels are concerned... but there is certainly scope for your solution in Chambers' definition for 'vowel' - ''a letter (usually a, e, i, o, or u)'', '...usually' could be Kcit's 'get out of jail card' but if it is, I hope it doesn't catch on, things could get pretty chaotic.
sorry - this has dragged on a bit.