I already know that using trip as an AoO against a foe while they're trying to stand up doesn't cancel their standing-up action, they still make it to their feet. However, Greater Trip permits an AoO when you make a successful trip, not when an opponent falls prone from standing. The developers have issued errata very specifically stating that these are separate events - separate enough to permit two AoOs against the same target if the tripper has both Greater Trip and Vicious Stomp.
If they're that dissimilar, and nothing in either the trip nor the prone descriptions say otherwise, then a trip can be successful (and thus proc an AoO from Greater Trip, though not Vicious Stomp) even if the foe is already prone, right?
I'm building a character with a class from a third-party book that gains an animal companion and can select class features to give their companion access to all their own Combat feats, which means I can get Greater Trip onto my wolf companion despite the Int requirement. I also have a combo in mind that uses the same principle of successful trips against prone targets using Relentless Shot to allow me more battlefield control, since I'm not exactly a damage dealer.