  We study the vacuum polarization of a massless minimally coupled scalar field
at the horizon of four-charge STU black holes. We compare the results for the
standard asymptotically flat black holes and for the black holes obtained in
the "subtracted limit", both in the general static case and at the horizon pole
for the general rotating case. The original and the subtracted results are
identical only in the BPS limit, and have opposite sign in the extremal Kerr
limit. We also compute the vacuum polarization on the static solutions that
interpolate between both the original and the subtracted case through a
solution-generating transformation and show that the vacuum polarization stays
positive throughout the interpolating solution. In the Appendix we provide a
closed-form solution for the Green's function on general (static or rotating)
subtracted black hole geometries.
