I am building a cottage in Northern Michigan and for the foreseeable future it will not be used in the winter. The walls are Tyvek wrapped 2x6 with fiberglas. Last year, I install interior poly for vapor control. When I returned there was condensation on the outside of the poly. Because of the insulation, the inside of house is colder than the outside and the condensation is forming in the insulation. I have been advised to remove all vapor barrier even though that when it is heated, the condensation will appear on the exterior siding. Since the siding is much less expensive than the interior pine, I assume that this is the only option.
Any other advice would be greatly appreciated.

Make sure you have adequate ventilation. It seems you did eveything right.

From your post, it seems that your walls are still unfinished. Until the walls (drywall, wood etc.) are installed, air will move around the poly, carrying moisture that will condensate. The wallboard will seal this, pinching the poly between the wallboard @ the plates & studs. Until that time, you will experience the type of problems you described.

The Insulator