It is IME (GA only) unusual to fly a STAR using own VNAV. Even if one is flying a named STAR whole, the vertical part is according to ATC direction. Most STARs do not have clear vertical levels assigned to the different bits.
And I cannot see why PRNAV is necessary for this. All one needs is the ability to fly a programmed track on autopilot.
Maybe PRNAV has been used as a stick to beat the "old hardware" airlines into upgrading their avionics, on the pretext that the improved lateral accuracy is necessary?