From my experience, you cannot use a UA voucher on any star alliance partner, but YMMV...Where are you based? One thing that was very convenient w/ transiting through YVR last year enroute to SFO was the fact that my partner could clear US customs w/ me in Canada, and he didn't have to wait in the enormous line for non-US passport holders. Of course that may not be an issue for you, but I found the whole clearing US customs in Canada thing to be very convenient.
I'm assuming you are coming back from IST at some point, which is why I'm offering the above information...