I was wondering if the UFIR rail with a vertical grip is compatible with an Insight M3 mounted directly in front of the vertical grip.  Can you actuate the toggle switch on the M3 light effectively?  I see that GG&G offers an "offset tactical light mount" for an additional $75.  Is this needed to use this setup?

Just for S&Gs, I slipped my sure-fire nitrolon pistol light on it. It might have worked out on a full size rifle, but not on a carbine. I was only able to activate the switch one way with my hand on the grip.

Yep, I have the exact same setup on my Bushy carbine. I swap both the M3 and M6 on a regular basis. At first they fit very snug on the UFIR, now they fit perfectly. I found the UFIR to be too short for my large paws, so I ordered a 6 inch rail from Brownells. This rail extends the entire 6 inch length of the carbine handguards. Now my left hand has plenty of room. My UFIR is now for sale for $35 shipped. The GG&G vertical foregrip is a fine unit. Since I prefer the vertical grip as close to the mag well as possible, the GG&G UFIR didn't cut it. I can activate the light or laser with my thumb or index finger. If you have a Glock with rails, the vertical foregrip will mount to the Glock's rail. It turns some heads at the range.

If I'm not mistaken, that makes the glock an AOW and you could get in trouble.