> I don't know what the hell Z-amplitude is, but the brightness on vector games is > simply a factor of how long the beam remained on a given spot
Vector games actually could modulate the CRT's cathode or screen grid like regular video and get different brightnesses of vectors (for Atari color vector games, this is how they selected the colors). The overdrive stuff like the shot in Asteroids was done by drawing it multiple times at max amplitude.