What's the units of pressure in that plot? Torr? mbar?
Why don't you look at the pressure in the air (not just at the surface of the ski). Follow some parcels of air along their streamlines and see if there are any for which the pressure stays sufficiently low for a long enough time to produce condensation in conducive conditions.
My guess is that because of the very small size of the vortices that you see from skis compared to those from aircraft, the max pressure drop will be much less and will last for a much shorter period of time before it equilibrates. Therefore, even with nucleation centers available, and favorible RH, while a vortex might form, vortices from skis won't be large and strong enough to be self-visualizing.
To support this argument, how many self visualizing vortices do you see from small UAVs compared to full size vehicles at similar speeds?
That is pressure coefficient, which is non-dimensional. It is defined as:
Cp = ( p - p_inf) / ( 0.5*rho_inf*U_inf^2)
(where p is pressure, rho is density, U is velocity, and inf denotes a freestream value). Cp gives the ratio of the change in static pressure to the dynamic pressure, and is handy for aerodynamics. Cp=0 is freestream pressure, Cp<0 is low pressure, and Cp>0 is high pressure.
It's hard to say if the vortices off skis are ever visible, but my guess is no since we haven't really seen any (especially now that this image has been partially debunked). I'd have to pull in some other analysis to get condensation worked out, but the Cp in the vortex core is -2 or so, which is inline with values seen on similar types of vortices on aircraft. I have seen vortices show up on small scale wind tunnel models, so it's not totally out of the question. It really takes the right combination of ambient conditions to be possible in almost any scenario.