  We study the phase-separation dynamics of a two-dimensional Ising model where
A and B particles can only exchange position with a vacancy. In a wide range of
temperatures the kinetics is dominated, during a long preasymptotic regime, by
diffusion processes of particles along domain interfaces. The dynamical
exponent z associated to this mechanism differs from the one usually expected
for Kawasaki dynamics and is shown to assume different values depending on
temperature and relative AB concentration. At low temperatures, in particular,
domains grow as t^{1/2}, for equal AB volume fractions.
