Thanks for the replies, going to try the Renaming!
Was the cable length, though. Lesson learned: those signals are not one-way. Ideally, a computer recieves a “Hey, I’m here, my valid resolutions are …” from the projector. But just not in a HD, longish VGA, shit-ass Vista Laptop scenario. So, yeah, it reverted to the last time it was explicitely told valid resolutions which was with a Triplehead.
Well, it’s a little fun installtion I wanted to do for a while and it’s in the window of an empty shop around the corner, so my solution today was to “fuck this, I get my real computer” :)