  In this paper we have shown that squeezed modified quantum vacua have an
effect on the background geometry by solving the semi-classical Einstein Field
Equations in modified vacuum. The resultant geometry is similar to (anti) de
Sitter spacetime. This geometry could explain the change of causal structure -
speed of light- in such vacua without violating diffeomorphism covariance or
causality.The superluminal propagation of photons in Casimir vacuum is deduced
from the effective electromagnetic action in the resultant curved geometry.
Singling between different vacua is shown not to violate causality as well when
the geometric effect on the null rays is considered, causing a refraction of
those rays when travelling between unbounded and modified vacua.
