I am planning a shop structure in the Sierras, above the snow line. I expect to be able to get footings and slab placed by fall of 2018 but would not be able to erect the building until spring of 2019. I plan to put 8" crushed aggregate, 2" of EPS foam and vapor barrier under the concrete. My concern is that the vapor barrier will accumulate water under the slab during that first winter and that trapped water will freeze and thaw numerous times during the winter. Is the vapor barrier likely to be a problem or am I worrying about nothing? thanks

If the vapor barrier is between the foam and concrete it's really a matter of how much moisture you allow to enter the concrete from the top. (If the vapor barrier is between the stone and foam there is the potential for trapping pockets of liquid water.) Putting a sheet of 10 mil poly on top of the slab before abandoning it for the winter, overlapping any seams by at least 12" and taping the seam with housewrap tape should control any freeze/thaw spalling issues.

Above the snow line in the Sierra would probably be US DOE climate zone 6 ,and could even be zone 7 if it's high enough. In zone 6 it would be financially rational on a lifecyle basis to bump the EPS to 2.5- 3" or even 4" if using reclaimed roofing EPS (which is dirt cheap-R). See Table 2, p10:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf