A study of the poly(vinyl methyl ether) (PVME) segmental dynamics of bulk miscible blends of polystyrene (PS) and PVME reveals that while at high temperatures, T, there is evidence of a single a-relaxation process, at lower T, two separate dominant relaxation processes, associated with the change in structure of the blend with decreasing T, emerge. One relaxation process decreases with a much stronger dependence on T and "freezes" at a temperature comparable to the glass transition temperature, T-g, of the blend measured using differential scanning calorimetry. The other exhibits a weaker T dependence and persists at much lower T, becoming Arrhenius (the so-called alpha'-process) at sufficiently low T. In thin PVME/PS films confined between aluminum substrates, a new relaxation process, ales, associated with PVME chains that preferentially segregate to the substrates, emerges. These observations are considered in light of the influence of spatial compositional heterogeneities on blend dynamics.