Stylus force digital scales

@nrenter wrote The only change in the vertical force vector is as the stylus is raised vertically is due to the change in gravitational force as you move [mm to cm] further away from the center of gravity, and therefore the force of gravity changes. This is why it's called "Vertical" tracking force - vertical states the direction of the force vector.

This statement is incorrect and you gave the reason in the body of your response. The force of gravity is at its greatest when perpendicular to the earth's surface (you call it vertical).  With a pivoted tonearm, the cartridge travels in an arc and within that arc, there can only be one position in which the stylus tip is perpendicular to the playing surface and it is in this position in which the force of gravity is at it's maximum.  Any deviation from perpendicular will result in a less gravitational force. 

This is the same rationale as why weigh scales must be place on a flat surface to read accurately, otherwise the weight will not be transferred perpendicular to the earth's surface.

All physically true, but, does it matter? The optimal VTF for any given cartridge is an unknown

I agree, I only use the stylus force gauge to set the vtf at the manufacture's recommended force and then perform fine adjustments by ear.  Personally, I don't need a scale that measures to the 1,000 of a gram.

Lewm, I believe we are saying the same thing if I replace force of gravity "on an abject" with force vector. An object perpendicular to the earth’ surface will experience only one force vector. Anything other than perpendicular, results in more than one force vector and that is (as you obviously know) why balls roll downhill and objects topple if not supported. Thanks for the additional clarification.