Is it possible to cast to the Vufine via a WIDI or Miracast dongle? Most of them seem to output 1080P. Will the Vufine negotiate with a dongle to receive 720P?
It's not up to the vufine to negotiate, it'll tell the device it wants a 1280*720 signal, and the device can either listen and provide one, or ignore the vufine and output at whatever it likes.
If you're keen on a specific device that ignores the vufine's request, you can use a 1080p to 720p HDMI scaler as discussed in another thread.