The ordering dynamics of the two-dimensional Ising antiferromagnet with mobile vacancies and nonconserved order parameter is studied by Monte Carlo temperature-quenching experiments. The domain-size distribution function is shown to obey dynamical scaling. A crossover is found from an algebraic growth law for the pure system to effectively logarithmic growth behavior in the dilute system, in accordance with recent experiments on ordering kinetics in impure chemisorbed overlayers and off-stoichiometric alloys.