  We review a long-standing difficulty in some semiclassical models of vacuum
and vacuum decay. Surprisingly enough these models, careless of their
transparent formulation, are affected by, both, technical and conceptual
issues. After proving some general results that are relevant for, both, the
Euclidean and Lorentzian sectors of their dynamics, we briefly highlight their
importance in connection with the issues discussed before, arguing that their
solution might be interesting in our quest for quantum gravity.
