A not too complicated solution:

Convert the midi tracks with these VSTi to audio.

Then the mixer pan should work just fine on the saved audio.

I would head over to the wishlist and add this one in. Here's my opinion for what it's worth - the mixer should work on AUDIO; whether it's recorded audio, or the output of a VSTi, it shouldn't matter.

You shouldn't be required to go through the steps of committing to audio before doing a little panning experimentation.

-Scott