well i think we all know where he did die,in the demon world,he's power was to much to much to control,so he sealed his power into his sword of the demon world,i'm not sure.he could have died from an assualt from a couple of strong demons or even mundus himself.
that's when vergil decided to stay in dmc3 so mundus must have did something to him,to possess him.and you'll know this if you killed all the 100 demons during the ending,you get a short bonus movie,showing vergil charging to some kind demon,which my guess is mundus.for two reasons.1)when you fight him in mission 22,and when your in the sky fighting him,he has some sort of fireballs that surround him,which looked similair to the ones that where in dmc3 and 2)this is a wild guess,but it could be mundus because you could hear the muisc from dmc1,and you don't hear any dmc1 music any where else in dmc3 exept from the jukebox you break in mission 3.
since sparda was getting old,some of that almight power could have been lost.and vergil could have met him in the demon world,maybea that's why he follows mundus in dmc1 because he and his father lost the battle.i believe this because in dmc1,when you kill vergil for the last time.you get his amulet.And when you get it,you combine it with your one to form the real true form of force edge.maybea that's why,when vergil got force edge in dmc3 in the last mission.it didn't turn into the sparda form of the sword,maybea he needed something from his father?
thnx koRn for lettin me use on of your sig's!