We propose an efficient and robust method to generate tunable vector beams by employing a single phase-type spatial light modulator (SLM). With this method, a linearly polarized Gaussian beam can be converted into a vector beam with arbitrarily controllable polarization state, phase, and amplitude. The energy loss during the conversion is greatly reduced and depends mainly on the reflectivity of the SLM. We experimentally demonstrate that conversion efficiency of about 47% is achieved by using an SLM with reflectivity of 62%. Several typical vector beams, including cylindrical vector beams, vector beams on higher order Poincaré spheres, and arbitrary vector beams attached with phases and with tunable amplitude, are generated and verified experimentally. This method is also expected to create high-power vector beams and play important roles in optical fabrication and light trapping.