Some MetPy functions, such as gradient and advection, allow the user to supply arrays of deltas rather than a single value, but vorticity (and its relatives) does not. Vorticity is more complicated since derivatives of the "map factors" are also required, but it should be doable.

Here is the necessary computation according to the GEMPAK documentation (Appendix B2):

Since the map factors (m_x and m_y) are equivalent to the "nominal" grid spacing (\partial x and \partial y in the above equation) divided by the actual delta, an additional "nominal_delta" argument to vorticity would allow for the map factors to be computed. Since the WRF model provides the map factors in its output, allowing the map factors (and the nominal grid spacing) as optional arguments to vorticity in lieu of deltas would be useful as well.