patriot, you don't list your location so I have no idea what you elevation is.
I had a 98 KX250 and a buddy of mine had a 99 that I helped get his running right.
You are right about your bike have a Keihin PWK with a power jet. My understanding is very few people mess with changing the power jet so you are basically looking at tuning it like any other carb.
The first thing I would do is double check your installation to make sure you don't have an air leak on the vforce. Vforce claims that you shouldn't need to re-jet if the bike was jetted properly before hand but it is possible you were a little off before. Jeff's 99 was bone stock and was a plug fouler when he got it. We pulled the carb and saw the needle came in the 4th clip. (counting down) We moved it to the second clip and never had another problem.
This was at 900ft elevation, mostly woods, C level rider.
I agree going up one size on the pilot and seeing what effect that has would be a good place to start, you certainly are not going to hurt it by doing this. Good luck, they are a lot more fun when they are running right.