  The response of vacuum to the presence of external conditions is the subject
of this work. We consider a generalization of the Casimir effect in the
presence of curved boundaries on which a sharp potential is concentrated. The
profile of the potential is a delta function, which has some features in common
with a hard boundary and some with a smooth background field. The boundaries
investigated are: i) a spherical shell, ii) a cylindrical shell, iii) a
magnetic flux tube. The vacuum energy is calculated by means of the Jost
function of the scattering problem related to the field equation. The energy is
then renormalized by means of a zeta functional approach adopting the
heat-kernel expansion. The heat kernel coefficients are calculated and a
discussion of the UV-divergences of the model is presented. The renormalized
vacuum energy $E^{ren}$ is then numerically studied and plotted. The sign of
$E^{ren}$ is found to be always negative in the case of the cylindrical shell,
while in the case of the spherical shell and of the magnetic flux tube the sign
depends on the value of the radius.
