How to make a diffuser for a 22 X 30 skylight shaft

If you do find condensation then you have a more significant problem that you would want to repair. Here's why: The drywall tunnel, being exposed to sunlight should always be hotter than your house, which means that relative humidity will be lower than in your house even if infiltrated with air from your house. That also means that any condensation would only be against the insulation barrier at the bottom. Unless you have other moisture infiltration into the tunnel that shouldn't happen. If it does then you need to insulate the tunnel from inside the attic or else find the leak that's letting moisture into the tunnel.

You do not want a vapor barrier on the cold side of the insulation. Switch your plan to use an air barrier (or an open mesh, since you should have little air movement in the space) and it makes more sense. On the other hand, if you already bought the plastic, just poke it full of holes (but do not poke through to the warm-side vapor barrier.)

But - this will leave you with essentially uninsulated areas at each stud, and compressed insulation near them - particularly galling since they are "flat-ways" (1-1/2" thick.) It would be better to attach ~6" wide boards to the studs (sticking out from the wall), so you have a nearly 6" deep cavity to fill with insulation, and no compression where the plastic or mesh is attached.

Edit post-comments: You could also fill in between the flat studs with 1-1/2" rigid foam cut to fit (and/or sprayfoamed at the edges), and then sheet over the whole thing with 1-1/2 to 2" foam covering the studs (with a bit more sprayfoam sealing any joints or gaps), but your question indicates that you already purchased materials, so I don't want to send you off on a mission to spend more money if it's not really needed. But if you'd rather do that than add boards, it should work fine - especially if you can return the materials you bought.