Controlled decend speed on long downhill

I am guessing you are talking about keeping constant speed using regenerative braking. This is a function of cruise control. You would activate the cruise control and it will keep the constant speed regardless whether you are going uphill, downhill or running on flat surface. It would modulate current  to hold specific RPMs you set and will regenrate as needed. At least, that's the way it's is supposed to work. VESC does have a cruise  function but I haven't played with it enough to tell if it works like that. I know my other non-VESC controllers do work that way and they are way less sophisticated. You can give it a shot. I believe cruise is activated on the SERVO input f you are using ADCs for throttle and regen.