The very most accurate wheel based method is to use 10 miles worth or more of interstate mile markers and this calculator to get a WS (wheel size) value that is very very accurate. This method is actually MORE accurate than the Savage speedo.
I don't know if your model of the Vapor uses WS values like the Sigma and the other bike type speedos do, but the tool is out there if it does.
http://tomstoys.50webs.com/bc80c.htmlIf your model of Vapor only uses GPS to do the trick, then you'll have to follow their instructions.