If it was Aqua's Armor then he would have known she was in the Realm of Darkness and he could have spoken to her eye to eye. If Xemnas really wanted to know where Ven was sealed, then he could have done so.
I think what you're saying here is that Xemnas could have found Aqua just by knowing that she's in the Realm of Darkness, and then she could have taken him to find the Chamber. I've seen this assumption made a lot, sometimes about Xemnas and sometimes about Mickey, and there's something about it that has to be said: the Realm of Darkness is
huge. It's not a "World," it's a "Realm." It's a
universe. The Realm isn't just the one or two rooms we've seen in cutscenes, it's everywhere. Knowing Aqua is in the Realm of Darkness is practically useless.
(I think the mistake a lot of people make is thinking The Realm of Darkness is just the shoreline from the end of KH2 and BBS, aka The Dark Margin? Afraid not.)
I guess the information is better than nothing. I mean, he cut his search area in half, from two universes to one. But it's still a universe. And the Realm of Darkness isn't divided into Worlds. Instead, it's pretty much full of weird causeways and and platforms, from end to end. As far as hiding places go, it's even bigger than the Realm of Light.
Sora and Co will have a lot better luck because Sora is tied to her heart, no doubt. Plus, she's not hiding from
them.