I have a Rifle-length YHM lightweight Free-float rail on my M4-gery.
I bought the Falcon Industries Aluminum vertical foregrip for mine. There was some concern that it MAY not fit my rails. As it turned out, it would NOT lock up on the rail.
The grip is locked up via a screw-up base on the VFG. This raises an aluminum screw that presses upwards in the center of the grip to strike the rail. On mine, the screw would not go up far enough to make tight contact with the rail.
I was about to send it back until I realized how easily I could create an adapter.
I used my dremel to create a polymer sleeve for the screw that had a closed top. I shaved the sleeve's closed top until I could get it into position. Once I did, I tightened up the screw. It locks up perfectly now.
If yours has a similar issue, don't dispair. Get creative.
Here is mine... Since this was taken, it now has a Millet DMS-1 1-4x22 optic on top. It also has an adjustable palm-rest grip now, but I don't know if I will keep it on this rifle.
-- John