You take advantage of the moment to brutally kick an enemy when he is down.
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 Roleplaying Game Ultimate Combat. © 2011, Paizo Publishing, LLC; Authors: Jason Bulmahn, Tim Hitchcock, Colin McComb, Rob McCreary, Jason Nelson, Stephen Radney-MacFarland, Sean K Reynolds, Owen K.C. Stephens, and Russ Taylor.