Nai Fohl
Advocatus Diaboli
Well...
My personal theory is that Vastor appeared to be so strong because he was a creature of the jungle and he was in his natural enviroment where Depa and Mace both were not.
But we never have seen him engage in a real fight with a Jedi (meaning a Jedi aimed with a lightsaber). He was good in aiding his natural abilities with force powers (strength, speed - he could deflect blasters) and he had developed some skills needed in his enviroment (control beasts and plants, heal jungle diseases) but neither were his abilities refined enough to deal with a Jedi nor has he developed some skills that would be useful in his enviroment (levitation, telekinetic abilities).
In the end Mace (being injured quite badly since Depa had pierced him with a lightsaber) defeated him by force pushing a vibroshield through Vastors own vibroshields and Vastor couldn't protect himself against that.
So while I agree that he would have become a powerful force user being trained by the Jedi or Sith I don't think he would be able to kill the "major" people of the PT order in a vibroshield vs lightsaber fight (while he would pretty much own all of them in unarmed combat).
And to answer the question why the Jedi didn't recrute more of the Korunai: There didn't seem to be many of them having enough force potential to become Jedi. With thousands of Korunai Vastor had only 8 (?) of them being his personal ******* and most of the Korunai didn't show any force potential at all (only communication with animals or each other through the force). I think Vastor would have created some real "army" of force users if he had enough people having the potential needed to develop the required force powers.