We report simulation results regarding the dispersion control in silica based microfibers, with subwavelength core radii. The results cover cylindrical waveguides obtained by tapering down a standard silica based optical fiber. As a strongly index mismatched clad layer, we study a polymer/resin with an refractive index tunable around 1.38 (through temperature or UV irradiation) or composed of the surrounding air. By analyzing a two layer vector model, we show how the dispersive properties of the waveguide, namely the zero dispersion wavelength and its slope, can be effectively engineered.