You can make free attacks on foes that you knock down.
Benefit: You receive a +2 bonus on checks made to trip a foe. This bonus stacks with the bonus granted by Improved Trip. Whenever you successfully trip an opponent, that opponent provokes attacks of opportunity.
Normal: Creatures do not provoke attacks of opportunity from being tripped.
The Greater Trip feat allows you to take an attack of opportunity against a foe that you trip. The Vicious Stomp feat allows you to take an attack of opportunity against a foe that falls prone adjacent to you. If you have both these feats and trip a foe, do you get to make two attacks of opportunity (assuming that you can)?
Yes, the two triggering acts are similar here but they are different. One occurs when you trip a foe. The other occurs when a foe falls prone. It requires a large number of feats to accomplish, but you can really pile on the attacks with this combination.
Pathfinder RPG Core Rulebook. Copyright 2009, Paizo Publishing, LLC; Author: Jason Bulmahn, based on material by Jonathan Tweet, Monte Cook, and Skip Williams.