  Any unified dark matter cosmology can be decomposed into dark matter
interacting with vacuum energy, without introducing any additional degrees of
freedom. We present observational constraints on an interacting vacuum plus
dark energy corresponding to a generalised Chaplygin gas cosmology. We consider
two distinct models for the interaction leading to either a barotropic equation
of state or dark matter that follows geodesics, corresponding to a rest-frame
sound speed equal to the adiabatic sound speed or zero sound speed,
respectively. For the barotropic model, the most stringent constraint on
$\alpha$ comes from the combination of CMB+SNIa+LSS(m) gives
$\alpha<5.66\times10^{-6}$ at the 95% confidence level, which indicates that
the barotropic model must be extremely close to the $\Lambda$CDM cosmology. For
the case where the dark matter follows geodesics, perturbations have zero sound
speed, and CMB+SNIa+gISW then gives the much weaker constraint
$-0.15<\alpha<0.26$ at the 95% confidence level.
