The VTF is not really affected by vertical effective mass, as long as the record is flat. IF there is a warp or change in vertical plane during play, then the effective mass can have an effect of compressing the stylus as it rides up the warp, and increasing VTF momentarily on the warp, and then being slower to come down the back side of the warp, causing lighter VTF on the back side. Lighter effective mass is better for this warp tracking activity, as it responds faster to changes in the record surface.
Optimizing the effective mass of the tonearm for the cartridge being used (mass/resonance matching) is the primary purpose of using tonearms of certain effective mass with certain cartridge characteristics. The purpose of my HiFi mod is to improve the matching of Rega and OL arms to lower compliance cartridges than they are optimally designed to handle. It increases horizontal effective mass for keeping the cartridge centered over the groove during play, with cartridges of low compliance that would normally drive the arm into unwanted movement laterally. It does not affect the vertical effective mass, so warp tracking is not changed from the standard version. The purposes of effective mass of a tonearm are to ensure that the arm is not driven into unwanted movement and stays centered over the groove when the cartrige is being excited by the groove modulations in the record, AND to ensure that any resonance in the arm/cartridge matchup is happening at a frequency which is least damaging to the audible listening range.
In many tonearms, the designers seek to make the effective mass equal in the horizontal and vertical planes. In other tonearms, there is more effective mass in one plane than the other. These are the decisions of the designer. In an arm with equal effective mass in both the horizontal and vertical plane, there needs no differentiation in the description or specs of the effective mass. In arms with different effective masses in the vertical or horizontal plane, many times only the vertical effective mass is in the specs. Only the Dynavector 507 series arms and the OL arms make a big promotional issue out of the two planes of effective mass, that I'm aware of.
Vertical effective mass can be measured with a scale.
Or it can be calculated.
Same with horizontal.