You can take a stereo output pair from your interface and run it into the aux in of your dtx 502. This would allow you to monitor the mix from the DAW and the sound inside the drum brain at the same time, but you would have to mute the tracks that have the drum vst on them.
I know this is an old thread but this is how I do it.
If you've found a better way that allows low latency monitoring of the vst while tracking I am all ears