maybe there's a way to use the VST plugin to do an offline conversion,
Yes, I have done that, but it's quite a process. Media Center is soooo cloooose to getting this right, and it would certainly be much more convenient to simply listen to existing media instead of doing an offline process.
So to Matt: Foobar2000's channel mixing and routing features are very flexible, but it can't do multichannel VSTs. Media Center can do multichannel VSTs, but its routing is inflexible. All Media Center needs is more flexible routing. If I could just specify which channels could go to which output (including none) and/or if Media Center could still process multichannel audio while still ultimately outputting to a different format (including throwing away channels) then it would be my ultimate media player.
So maybe in the DSP Studio "Output Format" page, if 2 channels are set on the output, then make it so that multichannel audio could be passed all the way through the engine (including multichannel VST plugins) until the final 2 channel output where there would be an option to just throw away the unneeded channels. Please consider implementing this.