Tunable, coherent vacuum-ultraviolet (VUV) radiation has been produced in the 1210-1290-A region by four-wave sum mixing in magnesium vapor. This important extension of the wavelength range of tunable radiation was achieved using KrF-laser-pumped uv dye lasers, whose sum frequency then directly overlapped the first odd-parity autoionizing state in magnesium. With powers of 10 kW in the fundamental dye-laser beams, a flux >/=10(8) photons per pulse was produced at Lyman-alpha, with higher intensities at the longer wavelengths.