Dawn --
You're right that Vasher obviously doesn't want to kill Vivenna. Then again, neither did Denth.
Instead, Vasher wants to use Vivenna to convince the Idrisese (or whatever) to do something (or not, as the case may be)...just like Denth did.
It is conceivable that getting the Idrisese to submit to the Halladrens is no more in their best interests than trying to start a war with them was. (The only reason it
could be is Vasher's assurance that the war is still avoidable -- which might or might not be true.) It's also possible that the only reason Vasher let Vivenna know the command to get her breath back was because she wouldn't have been a very convincing princess if she were still sick.
So it's still possible that both Vasher
and Denth are bad guys (though of course, Vasher hasn't yet clinched the issue by
kicking the dog, or parrot, as the case may be). I'm not saying that this is
likely, but considering Vivenna's recent experiences, she'd probably at least consider it. (Someone once said, "fool me once, shame on you; fool me twice, shame on, er, shame on you.")