  The conditions under which cosmologies driven by time varying cosmological
terms can be described by a scalar field coupled to a perfect fluid are
discussed. An algorithm to reconstruct potentials dynamically and
thermodynamically analogue to given phenomenological $\Lambda$ models is
presented. As a worked example, the deflationary cosmology, which evolves from
a pure de Sitter vacuum state to a slightly modified FRW cosmology is
considered. It is found that this is an example of nonsingular warm inflation
with an asymptotic exponential potential. Differences with respect to other
scalar field descriptions of decaying vacuum cosmologies are addressed and
possible extensions are indicated.
