This paper reports the results of Monte Carlo transport calculations for electrons in strong electric fields in silicon dioxide based on new choices for the deformation potential in acoustic phonon scatter and for the ionization threshold. Impact ionization is described on the basis of cross sections derived from many body dielectric theory coupled to a model insulator. The electron-acoustic and electron-elastic scattering rates are matched at 8 eV in order to provide appropriate limitation on the electron-phonon scattering. The sensitivity of secondary electron production to these parameters is described. Also described is the important role of bulk trapped negative charge in affecting secondary electron production by changing the local field value. The results show that suitable choices for these parameters lead to appropriate levels of impact ionization and vacuum emission spectra.