  We study the Casimir effect due to a massive vector field in a system of two
parallel plates made of real materials, in an arbitrary magnetodielectric
background. The plane waves satisfying the Proca equations are classified into
transverse modes and longitudinal modes which have different dispersion
relations. Transverse modes are further divided into type I and type II
corresponding to TE and TM modes in the massless case. For general
magnetodielectric media, we argue that the correct boundary conditions are the
continuities of $\mathbf{H}_{\parallel}, \phi, \mathbf{A}$ and $\pa_xA_x$,
where $x$ is the direction normal to the plates. Whereas there are type I
transverse modes that satisfy all the boundary conditions, it is impossible to
find type II transverse modes or longitudinal modes that satisfy all the
boundary conditions. To circumvent this problem, type II transverse modes and
longitudinal modes have to be considered together. We call the contribution to
the Casimir energy from type I transverse modes as TE contribution, and the
contribution from the superposition of type II transverse modes and
longitudinal modes as TM contribution. Their massless limits give respectively
the TE and TM contributions to the Casimir energy of a massless vector field.
The limit where the plates become perfectly conducting is discussed in detail.
For the special case where the background has unity refractive index, it is
shown that the TM contribution to the Casimir energy can be written as a sum of
contributions from two different types of modes, corresponding to type 2
discrete modes and type 3 continuum modes discussed by Barton and Dombey
\cite{1}. For general background, this splitting does not work. The limit where
both plates become infinitely permeable and the limit where one plate becomes
perfectly conducting and one plate becomes infinitely permeable are also
investigated.
