Why would you try to apply the triangle of velocities to the RF leg on an RNAV procedure?
Does anyone apply the triangle of velocities to a DME arc?
In both cases, the aircraft has positive course guidance and has instant feedback of any errors. All that is required is to correct the error and maintain the required (curved) track.
The simple answer is that on the perfect RF leg or the perfect DME arc there will be no difference between the required track and the track made good.
You could at any point check your heading and compare this to the track of a tangent to the curve at that point. This would be your (instantaneous) drift angle. At the same instant you could compare your TAS and GS. That would give you all the elements of the triangle of velicities for that instant. 1 second later they will be different.
Remember that a RF leg or DME arc are not the same as say the putbound turn in a hold/racetrack because in the hold/ racetrack turns there is no guidance and no allowance for wind can be made until the turn is completed.
if you want to for example calculate a time and fuel burn for a curved track eg DME arc then average the initial and final GS and fuel flow and calculate the length of the arc using basic circle formula. Must be goof enough for the short legs that normally apply.
Alternatively simply make a block allowance for the approach procedure (as most people do).
Why complicate things?