Chances are you're going to replace the foam if you take it to an upholstery shop anyway, (which is a great option), so when I still had my Warrior, I took the staples out, removed the vinyl, and sat on the foam. Then, knowing where the 'hot spots' were, I used a sharpie to draw circles, like a target. I used a floor-scraper razor blade that is about 6-8 inches long, bowed it a little and started slicing/scooping the foam.
Just a little at a time, then sit on it again, keep checking the feel. It will work just fine, it's cheap, (razor and 1/4" staples). Don't worry that it isn't perfectly flat, once it's covered back up with your vinyl, you wont be able to see that at all, and you'll be able to get a lot more time in the saddle before your @ss goes numb.
BTW, I get those long razor blades (thin and very flexible) at a carpet installers supply. They are very useful to have in the toolbox.