I am having a similar problem. I can plug in any monitor to the VGA port and it works fine. (Mirroring off) Works every time, I can even reboot with a monitor plugged in and all seems to work fine.
But when I plug in a projector I have problems. Usually my Mini9 just locks, no mouse, no keyboard, nothing. Occasionally I get vertical bars on both projector and internal screen, Unplugging projector causes screen to go back to normal, so it is a projector induced effect. These are not the same bars that I get if I turn on mirroring MirrorDisplays has no effect.
I have tried using the mirror friendly kext with same result (but I am not actually trying to mirror, I want to extend onto the external screen) Objective is to use Keynote or Powerpoint in presenter mode.
Very frustrating - this practically is the only thing my Dell Mini 9 'Mac Nano' can't do.
I see many posts about projectors causing lockup, but to date have seen no fix? Any ideas?