You're absolutely right to start this discussion now, however, for the moment we are not yet the strongest party. This will only come after we can actually do something with all the cities we are going to build.
For the moment, I would rather keep Vox around to split research, in that we can both research different paths of the tree. After that, a lot depends on exactly how weak they are going to become, and how much we can trust them.
One remark, though, I can hardly imagine that Vox does want to get such a submissive role, as our weaker brother. Because of that, I imagine that in a few dozens turns, Vox will be even less trustworthy than they are now. We should keep prepared for a surprise attack.