Well, during a quite bright day I set the screen dim to 0 and could still read it by going very close to it.

This means that your display doesn't have the "known problem" where there's a blown circuit. This is both a good thing (at least it's not faulty), and a bad thing (there's no obvious reason why you shouldn't be able to read your display).

Is it possible that when I replaced the amber screen with this non-AR blue I made some mistake and now it is not working properly and not giving me the full, optimum brightness?

Unlikely, unless you damaged some circuitry on the display board. If you were careful, you probably didn't hurt anything.

Also, in the faq it is said that " the VFD should be so bright that it hurts your eyes if you've got it at full brightness during the night. (...)" . That is not really my case. I ALWAYS have it at full brighthness, during the night, and it is just as bright as the rest of my car intrumentation. Maybe somewhat dimmer.

Assuming you've set it to 100 percent in "lights on" mode, this surprises me. I don't own your particular kind of car, so I don't know how bright its instruments are. But in my experience, these VFD displays are distractingly bright at night. When I owned the Mk1 (before they had dimmers), my wife made me turn off the visuals and text at night because it was painfully bright. Bright enough that she would hold her hand in front of the display until I switched it off.

___________
Tony Fabris