Dan,
I didn't know the VLT could be used as a sound card.Are you sure? I'll answer your question based on it being a sound card. OK here goes.
1.Set BIAB to use MME drivers with it's output to the VLT
2. Plug the guitar into the input of the VLT
3. Plug the thru/guitar out from the VLT into the IN of the Internal card.
4. Set the internal card to use ASIO. Might have to be ASIO4ALL.
5. Open AT3 in standalone
6. Set input to AT3 from the Internal Card/ASIO4ALL
7. set output from AT3 to Internal/ASIO4ALL
8. plug the outs from the Internal card and the VLT into separate channels of the mixer.
My mention of Internal/ASIO4ALL is I'm not sure what will be listed in AT3 for available drivers.Also I doubt your internal card will have ASIO so you'll have to use ASIO for all..

John
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