I know this can easily become a heated topic as some people are huge fans of the VFG, especially ex military guys who used them extensively and they have a good argument when it comes to a military perspective when they are humping the weapon for days on end. I also want to say that I am not against them at all and leave it up to the individual as a personal choice. If I teach a basic class or beginners, I prefer that they aren't used. Advanced tactical courses I could care less and I will even offer my preferred method on how they should be used. Many guys who are very experienced adapt and use them well.
Having said that, a novice should learn proper rifle mechanics, such as using the support hand, as just a support hand. The support hand should not be used to pull the weapon into the shoulder. It should only support the weapon and act as an index for a natural point of aim. Firing on F/A is another story when controlling recoil and I will the weapon into my shoulder using my support hand, so in this instance a VFG can be a plus.
On a proper thumbs forward technique with a pistol the thumbs point towards the target. A person has the ability to take their finger, point at something and be pretty much right on target, even when pointing with the thumb. For the pistol the thumbs forward helps to automatically get the muzzle in alignment with the target for quicker acquisition.
Now equate this to the rifle. This theory is widely taught in skeet or trap shooting also. On a rifle without a VFG, support the rifle from the bottom of the foregrip and index the pointer finger down the length of the weapon, in line with the barrel. This develops a solid natural point of aim. When you bring the weapon up on target the finger pointing in line with the muzzle will get you there quicker and with more consistency. The support hand should do just that, support the weapon and preferably not create any other unnecessary muscle tension or side to side or forward to rear pressure.
A VFG tends to make a shooter throw out basic fundamentals. When you have a grip, our natural reaction is to squeeze it. Causing tension. The other natural reaction is to use it as a lever to pull the weapon into our shoulder to help fight recoil or muzzle rise, which can be a bad thing (unless shooting F/A as explained above). A pistol should be gripped properly and your stance should also help to counter recoil but there will be a natural rise of the muzzle and you want the weapon to return to your neutral or natural point of aim. The same concept applies to a rifle but because of the mechanics, recoil etc, many people tend to ignore or throw out basic fundamentals that apply to both handguns and long guns. This can be even more prominent with people who use a VFG.
This is the way I prefer to see a persons grip while using a VFG. I like it even more when the index finger is on the foregrip and not the VFG. If you notice in the picture the thumb is indexed forward towards the target maintaining a natural point of aim like a thumbs forward grip on a pistol.
The above guy is Tulsa Police SGT Brian Hill. SWAT and firearms instructor. Also an instructor with the United States Shooting Academy (USSA).
As mentioned, I am not a hater or against a VFG and I don't want to cause a stir with anyone. It is a personal preference. If I were humping a rifle in the desert for hours on end day after day, I would probably chose to use one also.