I like the extra hand position the vertical grip offers.
When I am trying to find rabbits in the brush and have my gun at the ready for an extended period of time, it just feels better with the vertical grip.
I too brace my non trigger arm against my rib when shooting offhand for accuracy. I hold the bottom of the VFG while bracing my elbow against my ribs. I would use the mag, but why when I have a VFG? Also it is kind of muscle memory not to put pressure on the mag as that is what I learned from Big Army.
Lastly, the Vertical grip works awesome as a McGyver bipod in the prone position.