Bult4mud,
I'm sure you've noticed that the way longarms are used tends to be very different when shooting for accuracy or match style compared to the methods used when gunfighting.
Here, an HK trainer shows excellent form in the gunfighting stance.
[img]http://www.hkpro.com/umpshoot.jpg[/img]
Note the following:
1. Forward lean and squarer stance.
2. Shortened stock to allow proper cheek weld and eye relief from the sights.
3. Toe of stock placed higher up and closer to centerline, below dominant eye vs shoulder pocket.
4. Elbows down.
5. Off hand placed further back, around the magwell instead of the handguards.
This is kind of like the mod-iso stance for longarms. It allows fast movement in different directions and most importantly faster target aquisition and gunhandling during the fight.
Having the off hand around the mag well offers the same advantages of using the vertical grip. It basically put your support arm and hand in a stronger biomechnical position. It also uses more skeletal rather than muscular support so it cause less fatigue when used for long periods of time. Using the magwell in this fashion also helps control the muzzle when firing strings of rapid shots or when using full-auto. So in essence a vertical grip is not necessary and a change in stance and grip is all that's needed to get the same effect.
The vertical grip comes more into play depending on how your rifle is configured. As more accessories, optics, etc.,get installed the way you rifle balances and handles change. The vertical grip allows you adjust the feel of your rifle to maintain the advantages of the gunfighting stance described above.
On my rifle that I have the vertical grip placed pretty far back about 3" from the delta ring. This location was the sweet spot for my rifle where the weight/speed/balance was just perfect for me and yet still allows me to maintain a comortable combat stance.
Everybody is built differently and everyones rifle is also configured differently, so much of configuring a AR type carbine or rifle boils down to user preference. You basically just have to try different things and use what works best for you and your setup.