With the Cravers and the Hissho already breathing down your neck and demanding dust, I don't really feal the Vodyani should join them in this militarist style of play. 


I have an idea to make them feel different than cravers/hissho that I like to think of as "A deal with the devil."


The idea: Make the Vodyani strive for peace while in a cold war state. They should try to win over anyone they meet and form peace or alliances.


Once they win you over (or indoctrinate you into their faith) and you sign the peace treaty, they start making demands for dust, resources, or forcing you to provide them with population for essence boost.


Allow the Vodyani to leech while at peace or in alliance, and the more demands you reject, the more leechers they send you. 

Going back to cold war at any time causes them to become unforgiving, and causes them to want to be at war with you for the rest of the game, as long as they have the influence to pull it off.