Are you planning to use it as an auxiliary tank for you C? If so where are you going to mount it? Or is it to be used as a replacement? What kind of connection does the tank have on it now? Post a picture of the outlet if you can.
I am mounting it on the back.Building my own rack, everything will be bolted. It will look like a monster with two heads. I'm ordering a new petcock so I can run it easily.
Maybe look into how the ST1300 guys are tapping into the main tank, might be similar for the VTX. I know the engine is different but the fueling system may be similar if not the same.
The 1300 has a carburetor so it's much simpler than the ST. One top of that it is a vacuum petcock, but has an actual off position so it could be as simple as a tee in the line and turn off the VTX petcock and run the auxiliary tank until dry then turn on the VTX petcock and run it as normal. That's how I would do it.
This is how I plumbed a previous carbed Honda. I used the the auxiliary fuel cell as the primary tank and used the bike's own tank (draining via the reserve) as the secondary tank.