So, here's the problem: when Vuo outputs a stereo signal with
Send Live Audio to a multichannel audio interface, the audio signal is sent to all output channels.
Funny thing: the Yamaha can be set to two-channel output and then it does behave as expected, i.e. audio is sent only to channel 1 and 2.
Yamaha, eight-channel output
Yamaha, two-channel output