  This is the first of a series of three papers on open string field theories
based on Witten star product deformed with a gauge invariant open/closed
coupling. This deformation is a tree-level tadpole which destabilizes the
initial perturbative vacuum. We discuss the existence of vacuum-shift solutions
which cancel the tadpole and represent a new configuration where the initial
D-brane system has adapted to the change in the closed string background. As an
example we consider the bulk deformation which changes the compactification
radius and, to first order in the deformation, we reproduce the shift in the
mass of the open string KK modes from the new kinetic operator after the vacuum
shift. We also discuss the possibility of taming closed string degenerations
with the open string propagator in the simplest amplitude corresponding to two
closed strings off a disk.
