I am working on the 18th floor of a 22 story high rise. First we are putting down a vapor barrier (many, like 6 or 7, hardwood floors have failed in this building). Second, we are floating 1/4" QuietCore. Then, we are floating two 3/8" CDX criss-crossed and stapled together. Finally, we are nailing down 5/8" engineered unfinished to the floating sub-floor. One challenge we are facing is that we have to meet two round 3' stone medallions and one round 6' stone medallion. The medallions, I believe, will and should be bonded strait to the concrete sub-floor. That means that our floating sub-floor may bounce around the perimeter of the medallions. What solutions are there to securing the sub-floor just around the medallions.
We don't want to use fasteners because we do not want to compromise the vapor barrier.

Any ideas, solutions, and comments on the current plan would be much appreciated.

I don't believe you would have much noticeable movement; probably not a concern.
The bigger question is, you mention several failures in the building and you're particularly worried about the vapor barrier......what kind of failures have you had in the building? I would be surprised if they resulted from subfloor moisture. Many failures you see in high-rise construction result from LACK of moisture, lower relative humidity as you move up in the building. Humidifiers are almost always a must in those situations.

Obviously those tile guys will have to build up the substrate to your level so I would let them know what your going to do like a sample subfloor piece they can guage their height with.

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I, too, have mentioned that most likely it is an air moisture problem. I have thermometers in the condo and common hallway to document the relative humidity. I am and will stress that it is on the HVAC guy to have a meter to measure and keep the relative humidity at the right level.

As far as a solution around the medallions. I want there to be no possible movement against the medallion, I thought of doing the following.
Exchanging the 1/4" QuietCore for 1/4" Cork in the 2' surrounding the medallion. Glue the cork and sub-floor where the hardwood will touch the medallion. Then 2' out from the medallion, have my floating sub-floor meet the glued sub-floor, with an expansion space in between.
Do you see any problems with that?

That sounds like a good plan to me

Theoretically, if you fasten in one spot, in the middle/center of a floating floor, you would be OK.

Everything would shrink and swell off of it. Two opposite areas would not be trapped.

Why not set the medallion on top of the floating substrate? It can be done. Just glue it down with some urethane adhesive so it can move with the wood.

IMHO...Your risking grout issues on the medallian, from the movement. Also at the medallian where wood meets stone, even using siliconized grout.
I go for solid.