Tom, It sounds like it should work to me, but you don't say now much of the middle layer is insulation, and how much is wood. I don't think I would bother with vacuum bagging the 1/4" ply onto the foam. It's internal strength, especially tension would be quite low. I think I would run some cross rails at whatever interval you intend to fasten the T&G faces on with, so there will be more than 1/4" for staples or whatever you shoot through the tongue side with. It would be a lot easier to just use some liquid nails and staples to build that internal core and still have plenty of structural integrity. Just be sure to plan ahead, and not have any fasteners where you will need to drill for hardware later.
If you are using KD for the T&G, I'd plan on using a V-car siding type face with a little space to expand, because it probably will grow. However most Western Cedar around here is delivered green and full of water, so if it isn't KD, then make it as tight as you want, because it will shrink.
I would probably use a dot of mastic behind each plank by where the fasteners hit, and solid around the edges. The heavy mastic really sticks well to W Cedar. I've torn out some door jambs and side lights to replace with some I had custom made, and I literally had to pry it off, breaking it into little pieces about the size of hammer handle. I don't know who install that door, and had never seen the jambs glued solid to the cripples before, but it sure impress me as being stronger than needed.