I hope this is explained at some point. Because if the changeling had killed Jack then what would be the point of the trail for Vadik to find the ship and get Jack? This is the kind of (potentially) sloppy writing that takes me out of things.
That's assuming the Changelings give a shit. They're allied with Vadic, doesn't mead they are working for her. They led her to the Titan, probably as part do their deal with the portal tech. They probably don't care about Jack or her vendetta.