We have studied the thin film morphologies of polystyrene-b-poly(2-vinylpyridine) diblock copolymers and polystyrene-b-poly(2-vinylpyridine)-b-poly(tert-butyl methacrylate) triblock copolymers after "annealing" in the vapor of different solvents. We find distinct differences in the resulting thin film morphologies. The differences are explained qualitatively on the basis of concentration-dependent Flory-Huggins interaction parameters of the different components.