I have found a Roadhouse 2 into 1 exhaust in good condition. It came off a '99 Honda Shadow 1100. Could this possibly be adapted to an '03 Drifter 1500?
Anything is possible, but you most likely will need to either bend or cut and weld the header pipes. I don't know for a fact, but I cannot imagine both bikes having the same header configuration. If you have access to welding equipment, though, anything is possible.
Another possibility is to perhaps find a still-available set of 2-into-1 pipes made to fit the Vulcan, and then just adapt the roadhouse muffler to the pipes.