If Frozen is in the game and Vexen is not in charge of it, I'm not sure what world can be better fit. Scientific World like Big Hero 6 can kind of fit, but that is not the case either, so either his world is connected via an unforeseen connection or he is not in charge of a world (supportive research), in charge of an original world, or he is not really a seeker.
They did say three sets of three worlds, but not necessarily 9 (Disney) worlds, so I would say 8-10 Disney worlds is reasonable to expect. It depends on whether a mini-game world like 100 Acre Wood (if included) is counted as a world or not.
Alternatively, if the chess piece next to the presumed Larxene's piece is Vexen's, he might also be in charge of Frozen similar to Ansem/Xemnas.