1-1/2" of foam and a vapor barrier? Trouble. How's your wall assembly going to dry out? Might want to spend some time over at Lstiburek's site:
The typical detail for windows over foam is to block out, with wood, flush to the foam - then attach your window to that. Requires careful detailing of the drainage plane to prevent water intrusion at the seam between them. Either housewrap before, or over, the foam.
a few relevant links:
These other links, are from a pro-only forum, be careful about posting there if you're not in the trades; the forum regulars can be very mean. The building science sub-forum is open to anyone, as is the electrical. The rest of the forum... only pro's (as you'll see from the 2nd link, from the exterior details sub-forum, where the moderator closed the thread):