My name's Kel.
You are correct.
Since warm air flows to cold the warm conditioned air on the outside of the cooler warm will seek to migrate into the walls. Without a correctly placed vapor barrier the warm moist air will condense on the insulation and decrease the insulation's performance. There's an increased likelihood of mold.
You have to tell me more about the ceiling detail. You'll get the best performance if you separate the cooler from the larger building. Therefore they'll be something like a roof to the cooler which can be insulated. A vapor barrier will go over the insulation and be on the outside of the cooler.
Make sure to caulk and foam all penetrations.
Have I answered your questions?
Everything you say makes perfect sense. Insulating the truss area over the cooler , as well as preventing attic hot air from migrating to the cooler ceiling is difficult unless one would use a layer of solid expandable foam over that area. If not done this way I understand your concept of separation with a separate ceiling for the cooler room.
You could use something like Thermax between the top of the cooler and the trusses.
Thermax is polyisocyanurate insulation with metal foil on both faces. You could use several layers of the stuff that's around 40mm thick. The foil will act like a vapor barrier.
You could use expanding spray foam. I'd use the closed cell type.
Your size job could use a professional foam contractor.
If you wanted to do it inhouse you could buy a foam kit from www.sprayfoamdirect.com