[Spoilers] I THINK I KNOW WHY

[terra4eva]

you guys know how sora can wield the kingdom key right. well vanitas could wield that x- key and his halve of ven when they got together they were a whole. i think they joined. so because vens heart is inside and he could wield the x- blade. i think that sora is borrowing halve of vens power therefore giving him part of the x-blade which is the kingdom key

 

[Ventus Air-Man]

It's possible, but it is a bit of a stretch. Who knows though, I thought Vanitas's identity was a bit of a stretch.

 

[Shamdeo]

HOLY SHIIIIIIIT

 

[loke13]

you guys know how sora can wield the kingdom key right. well vanitas could wield that x- key and his halve of ven when they got together they were a whole. i think they joined. so because vens heart is inside and he could wield the x- blade. i think that sora is borrowing halve of vens power therefore giving him part of the x-blade which is the kingdom key

Riku was able to weild the KK.

 

[The Fishman]

Riku was able to weild the KK.

pretty much this, i don't think ven has anything to do with sora wielding the keyblade since it wasn't originally for him