What goes on here?

Is VAnitas fighting Ven on the inside while on the outside Vanitas in Vens body is fighting Aqua?

yes i believe this is accurate, the simultaneous defeat of vanitas is what brought ven to the state he is in now.

Yes, as you can see Vanitas was in control because Ven wouldn't fight Aqua and Vanitas is fighting Ven on the inside after they merged back together. The battles happen at the same time.

Yes, all three final battles take place at the same time. Terra's LS battles Terranort as Aqua and Mickey fight Ventus/Vanitas, while at the same time Ven and Vanitas battle in Ven's heart.

Ven is fighting Vanitas mentally. Aqua and Mickey are fighting Vanitas who was controlling Ven's body.

aqua is fighting this so called "venitas" while ven is fighting vanitas in his heart.

when ven beated vanitas, venitass x-blade goes nuts and exploded and transferred all four out of keyblade graveyard.

Actually I thought that Aqua fought with the controled Ven first and right after that Ven fought him in the inside. How's this? Because when Aqua fought Possesed Ven he fought with a complete X-Blade and Ven in the inside fought because Vanitas wanted to recomplete the broken X-Blade, which happened at the end of the battle of Vanitas and Aqua.

Actually I thought that Aqua fought with the controled Ven first and right after that Ven fought him in the inside. How's this? Because when Aqua fought Possesed Ven he fought with a complete X-Blade and Ven in the inside fought because Vanitas wanted to recomplete the broken X-Blade, which happened at the end of the battle of Vanitas and Aqua.

 

Nope, it wasn't fully complete, because Ven refused to be controlled. They happen at the exact same time. The moment Aqua destroys the X-blade is the moment Ven destroys Vanitas, which is why the X-blade breaks.

I always thought Aqua/Vanitas fight was first and then when the X-blade was going haywire it was from Ven/Van's fighting It works if it was the same time, though.

Well it doesn't quite say which battle occured in relation to the other battle, but it's hardly that important. They both took place in the same 10 minute timeframe, or however long Nomura intended it to be.