I have a sub-2k and the kel-tec rail to go with it. My problem is that I purchased a Daniel Defense fore-grip for the rifle, and it seems to not even be able to fit on the rail due to the rail being to close to the fore-end, I guess. It looks like I installed it right but I'd like a second opinion in the hopes that I didn't just waste my money.
I would say if that's the rail you want to use, 1. grind down the insert, it seems to be too thick. 2. the KT forend is polymer, so shave until it fits. And 3. this should be up in the Sub2000 thread, you'll get more answers when dealing with a specific model if it's in the proper arena.
Or you could put one of these on it, running back along the forearm. It will bring the grip further back and feel more ergonomic than that forward position. If you do a search for the part number (yhm-9473) you can find it for sale at less than retail in a lot of places. And yes the base mount is strong enough for VG use.
The DD foregrip may be molded extra thick depending on the gun they figured most customers would buy it for. I had a UTG foregrip which fit rather tightly on a plastic MagPul rail. I changed out that rail for a very compact metal rail and the grip wouldn't come close to sliding on. I took a belt sander to the top of the grip until it fit. The sanded part looks "abused" but when its on the gun you can't see that part. Just check for fit often so you don't take too much off.