You hold it like a normal pistol, by the pistol grip.
If you add a stock, you must register the gun as a short-barreled rifle ($200 stamp tax) - this will also allow you to add a VFG. If you want just the VFG and no stock, you must register it as an "Any Other Weapon (AOW)" which is a $5 stamp tax. If you do either of these without registering, you are indeed putting yourself in danger of being sent to prison on an NFA violation if caught.