What I don't understand about NVIS is how do you know it is skywave and not groundwave? I don't have much experience with 80m, but 150 miles is a distance that is quite possible for groundwave with 2m or 6m.
I think you are mixing your modes...
Groundwave propagation must be vertically polarized, and travels in contact with the ground.
Maximum range depends on soil conductivity and output power, but might run out to 100 miles or
so on 80m with high power, though signals would be weak unless you are over salt water. Coverage
drops with increasing frequency: you aren't going to get very far at VHF with ground wave. (Hint:
ground wave coverage would be just as good for a ground mounted quarter wave whip as for one
much higher in the air.)
Space wave, or direct propagation from one point line-of-sight to another. That's much more
common on VHF.
Most longer distance contacts on VHF that aren't line-of-sight are typically troposcatter or something
similar: signals are very weak, signals need to be propagated at low angles.
F-layer ionospheric propagation is most common at HF, at least for distances beyond groundwave
range. NVIS is a subset of this (though there is some evidence of paths via the E-layer instead on 80m)
that works when the operating frequency is below the
critical frequency. In this case, a low dipole
or similar
horizontal antenna) radiates (mostly) straight up to the F-layer and straight back down
to cover out to a few hundred miles. Because path losses are low, signal strengths are good. Generally
you have to change frequency as ionospheric conditions change over the day / year / solar cycle.
During the winter, especially with low sunspot counts, we typically use 80m during the day and 160m at
night to communicate around Oregon, while during the peak of the cycle during the summer we would
be using 40m during the day and 80m at night. The critical frequency rarely gets above 12 MHz or so,
and that only in the tropics, so for most of North America we only have 40, 60, 80, and 160m as options
for NVIS coverage.
With good sunspots the F-layer does occasionally support propagation on 6m, in which case things really
get wild! But generally the only ionospheric enhancement at VHF is due to the E-layer.