is vanitas after ventus' heart?

[Castaras]

when MX told him "Take what Ventus owes you and take Aqua's life" i know he meant kill Aqua, but what about Ventus? or is it about the battle they have at the end.. i don’t understand it

 

[Sephiroth0812]

when MX told him "Take what Ventus owes you and take Aqua's life" i know he meant kill Aqua, but what about Ventus? or is it about the battle they have at the end.. i don’t understand it

Yes, it is practically about Vanitas "absorbing"/enslaving Ventus' heart which is also a part of their final battles.
This is even reflected in the actions and dialogue in both Ventus' and Aqua's scenario, as Vanitas insists on completing their union/fusion not only because it would complete the X-blade (and that was the primary reason), it is also to firmly fuse his and Ventus' heart together (with him in control of course).
You have to remember that although both Ventus and Vanitas have each their own hearts (Vanitas' being a relatively newborn one), they originate from the same heart and at the core of Vanitas' heart is a shard that originally was part of Ventus' heart.
This is also why Ventus says this:

I've got a better idea. How 'bout I destroy you both?

He does not only want to destroy the X-blade, but Vanitas as well because he does not want to fuse with him and be dominated by him.

In Aqua's scenario it comes down to the same result, only more bluntly:

Vanitas: Correct. I am not Ventus. His heart has become a part of mine now.

Aqua: Shut up! I'm sick of your nonsense! Give Ven his heart back!

If you want to take it really specific you could even compare it to Ansem SoD possessing Riku in KH 1 and to Master Xehanort possessing Terra.
The only difference is that in these two cases the hearts involved were of different origin to begin with while in Ven's/Vanitas' case one of the hearts was born from the other originally.