With how close together we and Vox are, there are only three possible outcomes:
1) We conquer them.
2) We consign ourselves to a position of mediocrity because we don't have enough territory.
3) We find a way to work with Vox such that they're worth at least as much to us alive as dead.
If Vox keeps behaving in a way that fosters mistrust, they're almost certainly toast once we're ready for a quick and decisive war. But if they mend their ways to a point where we are willing to view them as an honorable and trustworthy partner, and if they are willing to accept a secondary role in what could be an extremely successful partnership, I think there's a way we could let them live.
My basic concept is that if two nations work together closely enough, they could research almost as one. The larger nation would do the researching for both, and the smaller one would buy those techs at a reduced cost, thereby providing gold to bolster the larger nation's research. As a result, the larger nation would be freed from maintenance and unit upkeep costs, and could channel 100% of its domestic income into techs and whatever luxury spending is needed. And it would probably have gold left over for rush buying that would help improve future income.
Similarly, in such an arrangement, the two nations would also behave as a single entity with respect to tech trading. Techs received in an exchange by one civ would be given to the other (or equal value provided if the other doesn't have the prerequisites), and any profits or costs in tech sales would be shared equally.
Further, because the two nations function as a single entity, they can make deals with still other nations to pursue different paths of the tech tree and exchange results. Thus, the deal with Vox would not inherently stand in the way of other deals in the future.
If such an arrangement could be made to work, it could be extremely powerful - especially if accompanied by a military partnership. (Imagine immortals and knights upgraded from war chariots fighting side by side.) And arranging such a relationship with a relatively weak civ would probably be much easier than arranging one with a stronger civ. So there may actually be a way we could let Vox live if they stop doing things that irritate us.