In a way, anchoring poles to the top of a footing, then framing the structure like a pole building is mixing two different construction methods. The 4' deep pole serves two puposes in a pole building; it gets the bottom bearing surface down below the frost line, and it also gives the structure lateral stability. Most pole structures use girts that the siding material is attached to. The shear transfer from the girts isn't the same as it is if the siding is attached directily to the poles, and there aren't as many. The 4' embedment of the poles helps stiffen the structure for lateral loads (wind). If you are using poles on 8' centers on top of the footings, some lateral bracing should be added to the walls to help give the building lateral stiffness.

As far as vapor barriers and insulation, there is a recent thread with a lot of discussion about this. If you are heating the building, a vapor barrier is recommnended. Insulating under the slab can be beneficial for retaining heat, but can reduce the bearing capacity of the slab based on the compressibilty of the insulation.

Any time lumber, even treated, is in direct contact with concrete, deterioration of the wood is possible. Concrete conducts water, and the lumber acts as a wick. The lumber will stay moist at the concrete almost all the time. The best thing you can do is isolate the two materials, even if you mount on top of a footing. Steel base plates with pole sockets work quite well.

Add some lateral bracing to the walls, base plates at the posts, and a vapor barrier under the slab and you'll have a good building.

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