The MHD equilibrium and stability calculations or stellarators are complex because of the intrinsic three-dimensional (3-D) character of these configurations. The stellarators expansion simplifies the equilibrium calculation by reducing it to a two-dimensional (2-D) problem. The classical stellarator expansion includes terms up to order epsilon2, and the vacuum magnetic field is also included up to this order. For large-aspect-ratio configurations, the results of the stellarator expansion agree well with 3-D numerical equilibrium results. But for low-aspect-ratio configurations, these are significant discrepancies with 3-D equilibrium calculations. The main reason for these discrepancies is the approximation in the vacuum field contributions. This problem can be avoided by applying the average method in a vacuum flux coordinate system. In this way, the exact vacuum magnetic field contribution is included and the results agree well with 3-D equilibrium calculations even for low-aspect-ratio configurations. Using the average method in a vacuum flux coordinate system also permit the accurate calculation of local stability properties with the Mercier criterion. The main improvement is in the accurate calculation of the geodesic curvature term. In this paper, we discuss the application of the average method in flux coordinates to the calculation of the Mercier criterion for low-aspect-ratio stellarator configurations. 12 refs., 3 figs.