I have good news. The previously mentioned Unity Tactical Direct-Mount VFG is now shipping! I have one in hand and can confirm that it is very, very good.
This vertical grip is compatible with KeyMod and most slick side “modular” rails. It comes with all the hardware you need to mount it to either. There are KeyMod vertical grips out there but this is the first vertical grip that I am aware of that is designed to attach to just about any slick free-float tube. Users of these tubes have been waiting for a grip like this for long time.
Check out Unity Tactical.