((I believe that the plan is to acquire the Morey and Slinkar, and any other enemies that the Vanguard have, as allies. So the wholesale genocide of the Morey is not currently being suggested. However, it is a war, specific targets are difficult to track, guaranteeing the safety of specific people is difficult and expensive at best. The major issue is whether or not to trust them. Both the Morey and the Slinkar seem to be outright scared of the vanguard. If the vanguard offered a cease-fire, informed the Slinkar and Morey of an enemy that scared the Vanguard, and offered technology in exchange for assistance against this enemy, then, well, it seems that the Vanguard could make an offer that would be too good to refuse. Combined with a promise to hunt down and destroy enemy leadership if hostilities continues it would likely put the forces perpetuating the hostilities in a position to accept or face opposition even from their allies...
Really, thus far, the Morey and Slinkar seem to have no clear idea of the Vanguard motivations, if the Vanguard truly wish to reach a diplomatic resolution to the conflict, it seems odd that there is not more dialogue. Particularly between the soldiers. The occasion in which we established dialogue with the Vanguard command prior to losing the system seemed to be a surprise to everyone and something that just didn't otherwise occur.))
Ask what they wanted with this star system.
Ask for a list of Morey vessels destroyed or captured when the star system was captured.
I do not really trust the Vanguard. Let's not forget what they did to Elianna's brood. It seems that there are a lot of decent people amongst the Morey who are going along with bad things due largely to a war that the Vanguard seem to largely be in control of. The loyalty probes seem to be largely based upon Vanguard technology, which doesn't say much for their ethical standards...
Ask them to let us leave, with a vanguard diplomat, to present their offer to our allies.