  The steady state of a vibrated granular gas confined by a movable piston on
the top is discussed. Particular attention is given to the hydrodynamic
boundary conditions to be used when solving the inelastic Navier-Stokes
equations. The relevance of an exact general condition relating the grain
fluxes approaching and moving away from each of the walls is emphasized. It is
shown how it can be used to get a consistent hydrodynamic description of the
boundaries. The obtained expressions for the fields do not contain any
undetermined parameter. Comparison of the theoretical predictions with
molecular dynamics simulation results is carried out, and a good agreement is
observed for low density and not too large inelasticity. A practical way of
introducing small finite density corrections to the dilute limit theory is
proposed, to improve the accuracy of the theory.
