I don't know the particulars of your bike, but IF you are picking up grounds and so forth inside the headlight, I would be tempted to run the "Hot" for the VM to the bulb feed itself (assuming that the headlight is "on" when the ignition is on!), other wise, as said above, find any positive that is turned on when the switch is on. You could do this right at the switch with a little ingenuity etc.
FWIW, your voltmeter, if you are picking up voltage in the headlight bulb and not at the battery, will help in showing a "relative" voltage... i.e. it may in fact be lower than the battery voltage because of wire resistance etc, but you can still use the information to good advantage... if the voltage is lower than what you normally see, there is something going on. In other words, don't take the vm reading to be absolute gospel until you have compared it to the battery voltage, then you will know what the difference is.... it could easily by several tenths of a volt, or more, different.