SOLVED (Well, sort of)
Read in Acewell motorcycle computer install manual that if getting erroneous RPM results when connected to coil, to install a 1M Ohm resistor in the circuit. So that's what I did. I spliced the red line and soldered in a waterproof resistor (remember that I have my red tach lead soldered into the black/red wire off the coil connector). Well, that gave me nothing on the Vapor. So I began experimenting. I figured I added too much resistance, so I jumped the resistor so I'd have a direct connection. The RPMs went haywire. Then I noticed that if I just moved the wire I was using to jump the resistor near the line on the other side of the resistor, I got RPM readings that looked good and stable.
So, my solution was to solder in a wire from one side of the resistor and wrap it around the red wire on the other side (I did strip the end of my jumper wire before I wrapped it around the red Tach lead). See the picture below. It's ugly, but it works. I suspect there is a simpler solution using a smaller resistor, but I only had one size.
I have about 30 minutes on the bike since I got the tachometer working and it's still working great. I'll provide an update in the future if something goes wrong.