A scanning tunneling microscope was used to study the ionization of single Mn acceptors in GaAs(110). The ionization state switches when the GaAs valence band is bent across a Mn acceptor level. This produces a ringlike feature in STM images, whose diameter depends on the tunneling conditions and distance to charged arsenic vacancies. By varying the latter, we could tune the ionization switching, as well as quantify the contributions from tip- and vacancy-induced band bending.