Huh?
I know "vig," but never've heard of PITA(except the bread). 'Splain, por favor.
PITA is, as Lon explained on p1, "pain in the arse" (or "ass", west of Ireland). Someone saying "pain in the arse" is trying to be rude to or about someone/thing. Reducing it to a set of initials takes away the rudeness. So it's a euphemism. I don't usually do euphemism. I'm much more likely to call it a fucking shovel. Which is why it didn't immediately occur to me that PITA meant "pain in the arse". See?
MG
Not really. I don't see how using initials, common as it is on the net, is one thing then another. I mean, WTF?