  We propose a method for calculating vacuum fluctuations on the background of
a spherical impulsive gravitational wave which results in a finite expression
for the vacuum expectation value of the stress-energy tensor. The method is
based on first including a cosmological constant as an auxiliary constant. We
show that the result for the vacuum expectation value of the stress-energy
tensor in second-order perturbation theory is finite if both the cosmological
constant and the infrared parameter tend to zero at the same rate.
