I don't think you can, at least easily.
I try to do a 3-thrids soft. architecture to reroute VLC to Audirvana + through iTunes & using Audirvana+ as integrated to iTunes.
The idea was to stream the audio from Vlc to iTunes through the network. That part works (with the need of recode the stream, which means degrading the SQ).
However, even if A+ is launched and set up as integrated to iTunes, it doesn't capture stream, whatever stream it is (neither from vlc nor from a webRadio...)
On other way to do it would be to do the "iTunes integration" of A+ for VLC, but That, is another story, and without the source code of A+, I don't see
even if we find out how it works in details, how to implement it above A+ & VLC itself.
Also you can split your movie into two files (audio + video), and ... try to start playing them, one in A+, one in VLC... in the same time !