Line-shape analysis of O-17 NMR spectra is used to probe vortex melting and dynamics in aligned powders of YBa2O3O7. Vortex transitions are identified by comparing their dynamics with the NMR time scale. Lineshape changes indicate a well-defined melting transition at a temperature, T-m. Below T-m there is a coexistence regime of solid and liquid vortices with a lower bound, T-g, which marks complete vortex freezing.