I carry a G27 as a back up and off duty at times. I started with a Desantis Insider, IWB, holster fit for a keltec .40. It is leather, very thin leather, with a metal clip. I already owned this and gave it a try. The 27 just fit inside it with the trigger guard just barely covered. I ran the clip through the upper strap on my vest and put the lower strap across the outside of the holster. Worked fine but the metal clip wore through my uniform shirt in 3 months.
I then switched to the 5.11 vest holster. It is a square, semi rigid, piece of nylon with the loop half of velcro on both outer portions of it and a soft "holster" shape hook half assembly.
This is very versatile. You can put this on the front of your vest or on any portion that has exposed velcro material. I did not like mine up front. I had my tailor sew two, 2" portions of velcro onto my vest straps and now the semi rigid portion is attached partially the front section of my carrier and partially to the lower strap. the upper strap is then pulled over the outside of the "holster."
You can also use this for on or off duty carry if you are wearing pants or shorts with cargo pockets. The semi rigid portion is the exact size and shape of a cargo pocket and I have used it several times this way since I've been on vacation.
The 5.11 cargo style/BDU style duty pants have a cargo pocket on both legs and I know one officer who carries his back up revolver in that pocket with this holster.
Hope this helps.
I never like the Uncle Mikes vest carrier holster. I bought it and never used it after I realized I could use the Desantis. The 5.11 wears better, is more functional, and is cheaper than the Desantis. It costs more than Uncle Mikes but is still not expensive.