Hello, I'm installing the windows in my double stud wall house (12"). Am I correct in thinking that the large rough opening should be considered like an horizontal wall and should have a vapor barrier installed on the horizontal jamb just like if it was a wall ( an horizontal wall...)?

Whether or not a vapor barrier is necessary or even desirable, as well as where it's located is highly climate dependent.

I'm talking a about a 9000hdd climate. The walls have a 6 mil vapor barrier on the warm side. My logic is that the jamb is the warm side and since it's large, it's like an horizontal wall...

It needs to be air tight, and vapor retardent, but doesn't need to be as low-permeance as 6 mil poly.

Anything under 1-perm will do- an inch of closed cell spray polyurethane, or even inch-thick wood is good enough.

If it's not air-tight, it doesn't matter what it's vapor permeance is- air-transported moisture of a quarter of a square inch of air leak would swamp the vapor diffusion numbers through even half-inch plywood.

Better than 6 mil poly on the walls would be a "smart" vapor retarder such as Intello Plus, or Certainteed MemBrain, which would offer more than an order of magnitude faster drying speed when things warmed up in the spring, and the assembly could dry in either direction. During the winter when interior air relative humidity is under 35% the vapor permeance is less than 1 perm, which is plenty of vapor retardency for an assembly with rainscreened siding. If the double studwall is filled with cellulose you'd likely be able to skate with NO interior side vapor retarder- depending on the climate particulars. (9000HDD distributed over 6-8 months isn't the same climate as 9000HDD over 11-12 months, etc.)

hello, I made a drawing of what I did. A worker working on something else saw that and said that it was a bad idea and the poly should stop flush with the window on the inside and not be under and around the window frame... this can be fixed because it's not foamed yet. But I don't see the problem...
http://inspectionpremierchoix.com/?attachment_id=285
p.s. I made an error in the drawing. there is no flexible flashing at the bottom of the window ( only the rough opening is flashed) so the poly is not tape at the bottom

Whether the surface is vertical or horizontal doesn't matter. If it's warm, then that's where you put the vapor barrier. And always pay careful attention to air sealing - with barrier, gaskets and tape, preferably on both sides.

and, around the windows, where do you consider the warm part to stop? On the outer edge of the window or on the interior side?

I would think that ideally the vapor barrier would stop at the center of the glass. But I don't know. Or know that it makes any difference.