VDOT is essentially just a different way of expressing your finishing time(s).

Despite the somewhat misleading name (V-dot-O2-max) should not be in any way conflated with laboratory-measured VO2Max.

In simplistic terms, the former is essentially the output of the latter, with athlete-dependent fitness, running economy, biomechanics and race execution dictating the degree to which one translates to the other.

To circle back round to the OP, the Firstbeat/Garmin method tries to model VO2Max using HRV measurements and then uses a system similar to VDOT for it's race prediction times; these times aren't a direct copy of the original Daniels and Gilbert paper (and probably rightfully so, given that study used elite middle and long-distance runners as subjects) but evidently a similar method.