  Recently different regularization schemes for calculations of the vacuum
energy stored in the zero-point motion of fundamental fields were discussed. We
show that the contribution of the fermionic and bosonic fields to the energy of
the vacuum depends on the physical realization of the vacuum state. The energy
density of the homogeneous vacuum is zero irrespective of the fermionic and
bosonic content of the effective theories in the infra-red corner. The
contribution of the low-energy fermions and bosons becomes important when the
coexistence of different vacua is considered, such as the bubble of the true
vacuum inside the false one. We consider the case when these vacua differ only
by the masses of the low-energy fermionic fields, M(true)>M(false), while their
ultraviolet structure is identical. In this geometry the energy density of the
false vacuum outside the bubble is zero, rho(false)=-P(false)=0, which
corresponds to zero cosmological constant. The energy density of the true
vacuum inside the bubble rho(true)=-P(true) is proportional to -
Lambda^2(M^2(true)- M^2(false)), where Lambda is the ultraviolet cut-off.
