You are indeed correct that I am seeking carry modes for a second handgun as a third weapon. It's quite SOP down here in FL and with other groups that I run across. I see it as being of primary significance to someone using a handgun as a primary. A breacher could conceivably be carrying a breaching shotgun, then transitioning to a handgun. This would make the secondary handgun a good idea here too, b/c the handgun is basically the primary entry weapon.
I've been using the Safariland 3004 tac holster for quite a while now, and am slowly getting around to getting a 6004. An interesting new development on the 6004 is the enlarged leg shroud with modular pouches. With a drill and some ingenuity, one could conceivably attach just about anything to one's leg harness. You're right in that just about every vest mounted holster seems to be a total afterthought as far as its design.
Thanks to all for the input, it was helpful for me to get more feedback from other likeminded individuals. It seems that I haven't really overlooked any carry modes for the secondary handgun. It would also seem that the market for a quality vest mounted holster is pretty wide open at this point.