First off, vapor lock cannot exist in a fuel system that is vented to atmosphere, which ours is. If fuel vaporized in a fuel line or passage on a TW it would simply flow up the fuel line into the tank. It is difficult to run a siphon system from an auxilary tank to the carb because air bubbles often find their way from the float bowl, past the needle and seat, and into the fuel line. That is why a fuel filter on its side will always have air in it.
Sounds like a fuel flow problem. If the screen under the lever in the petcock, the lines, and the carb are all clean, it can only be one of four things. First, make sure the fuel line is not kinked. Second, make sure the lines to the charcoal canister are not kinked or plugged. Third, pull the lines from the canister and run the bike. If the problem recurs, the canister isn't the problem. If the problem goes away, the canister has a flow problem. Fourth, there are screens hooked to the petcock inside the tank that can be stopped up. Remove the petcock to check them.