The phase diagram for the vacancy-ordered structures in the substoichiometric TiC_x (x=0.5-1.0) has been established from Monte Carlo simulations with the long-range pair and multisite effective interactions obtained from ab initio calculations. Three ordered superstructures of vacancies (Ti₂C, Ti₃C₂, and Ti₆C₅) are found to be ground state configurations. Their stability has been verified by full-potential total energy calculations of the fully relaxed structures.