  We investigate a systematic formulation of vector-tensor theories based on
the effective field theory (EFT) approach. The input of our EFT is that the
spacetime symmetry is spontaneously broken by the existence of a preferred
timelike direction in accordance with the cosmological principle. After
clarifying the difference of the symmetry breaking pattern from the
conventional EFT of inflation/dark energy, we find an EFT description of
vector-tensor theories around the cosmological background. This approach not
only serves as a unified description of vector-tensor theories but also
highlights universal differences between the scalar-tensor theories and the
vector-tensor theories. The theories having different symmetry breaking
patterns are distinguished by a phenomenological function and consistency
relations between the EFT coefficients. We study the linear cosmological
perturbations within our EFT framework and discuss the characteristic
properties of the vector-tensor theories in the context of dark energy. In
particular, we compute the effective gravitational coupling and the slip
parameter for the matter density contrast in terms of the EFT coefficients.
