Here's what I put on my Harley to get some more range:
It's a 2 gallon tank from tourtank.com
I put an on/off valve at the tank and Tee'd it in to my fuel line. I can run off the main tank, the aux tank or both. Like Elmoreman said--it will draw a lot of attention. People stopped me all the time and asked me if I had beer in it. Heck, I got pulled over by a cop for speeding one time and he asked me the same thing. "Why, yes officer, in fact it is beer. I run a hose over my shoulder so I can drink 100 degree Budweiser while I'm going down the road."
You shouldn't have any vapor lock issues--the system will work fine. The main problem is the ones that bmwblake mentioned. It takes up luggage space. I didn't have any issues with a 'more complicated system'. I got quick disconnects that seal when separated but never bothered to put them on. I think it weighs 4 or 5 pounds, so it adds a little weight but not a ton.
after some experience with an aux tank I'd say:
pros: cool factor, may be cheaper than a new tank
cons: loss of luggage space
I loved how folks would come up and ask me about it. The 2 gallon tank is $120 so maybe it'd save you some money. If money is not an issue a larger tank is certainly more practical.
I took a road trip from Alaska to Georgia with a 3.6 gallon tank. 16L is fine in the US. I'd get a 1 gallon Jerry Jug and strap it on some place. No sense buying more junk if you're not planning on taking long road trips on a regular basis. Heck, even if you are. Mega tanks aren't necessary. Hmm, so why do I have an 11.3 gallon HPN tank on my bike right now.....