A color can be dark and very saturated or light and unsaturated. And a black image will make a dot right in the center, as will a white image. But brightness levels will not show on the vectorscope. Brightness can only be measured on the waveform monitor. Today It is quite common for the waveform monitor and vectorscope to be combined into single unit that can switch between the two functions.
Some units even allow for the two functions to be superimposed.
Video Cassette Recorders are unable, on their own, to play back a signal that is stable enough to transmit, or cut into another video signal without break up. A TBC is used to synchronize the tape machine with the other signals in the studio. All the cameras in the studio, the character generator and the special effects generator are all synchronized or genlocked to a master sync generator. If they aren't, the picture will break up and roll every time a cut is performed, and dissolves and fades would be impossible. TBC's have an additional function.