Vertical grips don't work that well on KSG's and I wouldn't recommend one. First of all most vertical grips have very little contact area and your slide (picatinny rail) is pretty much plastic and they have been known to snap off .....and second it puts your slide in sort of a bind when actuating. If you notice your forend moves almost effortlessly when moving rearward and then fully forward. After a vertical grip it actually will become more of an effort thus contributing to the first possible thing that you don't want to happen. I use this grip
Fab Defense PTK Blk Black Ergonomic Pointing Grip Angled Foregrip w Storage | eBay
Also the extended selector is the best mod yet and they only sell them on Ebay as well
Kel Tec KSG "Extended" Tube Selector Switch by Hi Tech Custom Concepts | eBay
The KSG is one of my all time favorite guns that I own as there's nothing else like it hands down !