I've never used it and I consider it to be cheating (any amount, even as a last resort). It seems like either a bug or a trick the programmers put in to allow them to kill monsters easily but then forgot to remove for the final release. Anyway it's cheap. I did use X-Zone against Wrexsoul and doom against Intangir but without casting vanish (of course you don't need to here) and without knowing about v+d. Doomgaze is annoying but it's not meant to be beaten in one session anyway, so I never felt like using v+d against it.