  We consider a very simple toy model for a spatially varying `cosmological
constant', where we are inside a spherical bubble (with a given set of
cosmological parameters) that is surrounded by a larger region where these
parameters are different. This model includes essential features of more
realistic scenarios with a minimum number of parameters. We calculate the
luminosity distance in the presence of spatial variations of the vacuum energy
density using linear perturbation theory and discuss the use of type Ia
supernovae to impose constraints on this type of models. We find that presently
available observations are only constraining at very low redshifts, but also
provide independent confirmation that the high red-shift supernovae data does
prefer a relatively large positive cosmological constant.
