Your first sentence is the bottom line. There is no commonly accepted definition. Period.

Your last sentence is no help, unless the definition of a Vehicular Cyclist is "anybody who practices ANY technique or practice claimed by anybody to be a VC concept".

I've seen night lighting, "good attitudes", dynamic swerving, mirrors, lights, bright clothing, following every law, taking formal classes, studying books on bicycling technique, avoidance of mistakes and everything else under the sun put forth on the BF as "VC concepts" by the VC proselytizers.