Something I've been thinking about lately: Why would Xemnas place Marluxia out of all people as the head of Castle Oblivion? Marluxia was one of the newer members when he was placed in charge of the castle, why would Xemnas put a newbie in charge instead of a senior member like Vexen?

Marluxia was strong, and his leadership abilities obviously led him to be Lord of Castle Oblivion. However, Xemnas knew that there were traitors and thus sent them to Castle Oblivion, why? Because of Sora and Riku. Xemnas has always seemed one step ahead and probably knew that the duo would end up at CO, and also knew that they would be able to annihilate anyone that stood in their way, which, with the help of Axel, they did. Want to get rid of traitors, send them to an isolated Castle to meet their doom...that's my theory.

 

actually the people of CO already knew about Sora and Riku . at the beginning of chain of memories Marluxia led Sora there and taught him how to fight using the cards.

The entire C.O. Operation was just a cover to dispose of those who had the potential to get in the way Xemnas's true objective

1.Marluxia didn't want his heart back.

2.Larxene same

3. Axel was a loose canon

4.Vexen was just plain annoying.

5. Zexion never trusted Xemnas. (Watch Blank Points, you'll see)

6.Lexaeus was only loyal to Zexion.

 

All in all Xehanort and all of his incarnations have proven to bee masters of the Xanatos Gambit to the point of Omniscience.

 

As for your question likley because he was third in the chain of command where as Vexen had the lowest of the 13 thrones.