The coherence of very long baseline interferometry (VLBI) at millimeter wavelengths can be improved through the use of a water vapor radiometer (WVR) to estimate the atmospheric path length variations along the line of sight to each antenna. Measurements of the water vapor emission spectrum using a simple scanning WVR have been used to correct the interferometer phases, thereby lowering the rms phase variation and increasing the coherence time. The average improvement in the coherent fringe amplitude of 400 s VLBI scans at 3 mm wavelength was 20% with a general trend of a greater improvement for the scans with larger phase variations. Sampling of the spectrum from 18 to 26 GHz allows separation of the water vapor emission from instrumental effects and from the emission of water droplets in clouds. The observed line profiles favor the Van Vleck-Weisskopf line shape.