  In the presence of a short-distance cutoff, the choice of a vacuum state in
an inflating, non-de Sitter universe is unavoidably ambiguous. The ambiguity is
related to the time at which initial conditions for the mode functions are
specified and to the way the expansion of the universe affects those initial
conditions. In this paper we study the imprint of these uncertainties on the
predictions of inflation. We parametrize the most general set of possible
vacuum initial conditions by two phenomenological variables. We find that the
generated power spectrum receives oscillatory corrections whose amplitude is
proportional to the Hubble parameter over the cutoff scale. In order to further
constrain the phenomenological parameters that characterize the vacuum
definition, we study gravitational particle production during different
cosmological epochs.
