  Using the formalism of the interaction picture we calculate an expression for
the Wightman function for only the spherically symmetric modes of a quantum
Klein-Gordon scalar field in a general Vaidya spacetime with ingoing null dust.
It is demonstrated that particle detectors following time-like trajectories
that are in the ground state at some time outside of the collapsing shell will
respond independently from the configuration of the ingoing null dust if the
response is taken at any time outside of the collapsing shell. For detectors
that are taken to be in the ground state at a time interior to the shell it is
shown that their response will depend on the configuration of the ingoing null
dust. Relevance to the information loss paradox is discussed.
