The description of the Allip in the temple of Vecna in TFoE has it wearing wizard's robes, but the Allip is "incorporeal". Please correct me if I'm wrong, but doesn't that mean it can't wear clothes unless it has the manifest ability (which it does not)?
Also, the MM says the Allip is constantly wandering around muttering to itself with its "babble" ability. Would this be a problem for the acolytes and faceless one, or are we assuming that they keep making their Will save every day, or are we assuming the Allip is _not_ "babbling" all the time like the MM says?
I think that you can just describe the Allip's incorporeal garb as being wizard's robes. An Allip wears robes as it is (if I recall the picture correctly), so just add some color and make them wizards robes.
As far as the will save - clearly [brace yourself, I'm gonna start making stuff up out of whole cloth] the arcane residue laced within the walls is what created the Allip. Since the worshippers of Vecna are immune to that kind of stuff, they must be immune to the Allip. Or they just get a +4 circumstance bonus on their will saves. Or something.
My players encountered the Allip first, and took care of it first before heading in to deal with everyone else.
Well, the PCs faced the allip and acolytes last night. I decided that the allip could simply "turn" the babble ability on and off as a free action by choosing whether or not to make any noise at all.
The PCs HATED the allip! They were scared badly by it. One of them lost 8 Will, and two others each lost 4! (I rolled a lot of 4s) They got the allip down to very low HP and it went to hide in the wall. The PCs don't want to leave without killing it first because they are afraid of allowing it to escape!