I don't know if this will work but just an idea for you to explore. Check out the Roland DIF-AT box. Occasionally see these on eBay. It may be possible if you sent the Tascam signal into the DIF-AT which will send it out to the RBUS cable which with some custom wiring you could loop back into the same box which should then send the same signal both to the TDIF and the ADAT port. It is easier i think to find an interface that accepts ADAT e.g. Motu.
For the above you will also have to find a way to supply 5V power to the DIF-AT as it would be powered from the Roland equipment via the RBUS cable.
If you are serious about trying this then there is useful info on the VS forums and i may be able to give you some info on RBUS. Roland also could give you the schematics for the DIF-AT if you ask them nicely.