I don't know about the Vapor unit, but on my Voyager tach input, I connected it at the ignition coil primary. This is the spade terminal on the coil itself. Trailtech's instructions said to wrap the wire around the high tension lead, but I wanted something a bit more solid for a connection. I verified this method with them before trying it. I did have to set the display to "tach/2" to get the correct RPM reading.
It sounds like you are referring to the power wires in your posting. Do you not have a separate wire just for the tach input? Check trailtech's web site for the instruction page (download section).
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2011 EC250E
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