Though generally not considered as 'mission critical' during shooting as exposure scopes such as Waveform & Histogram, the Vectorscope plots the chromaticity, or the hue + saturation of your image onto a color wheel so you can help gauge complimentary color, ensure skin tones are being colorized properly, to visualize how overall scene color is being placed, and where potential adjustments may need to be made in general.Vectorscopes are also used in Broadcast to make sure that the colors of the footage are exact. Vectorscopes will have a target for each zone. The idea would be if shooting a pure sample (such as a color or macbeth card) that the targets would be hit. You will see vector scopes used in post production to be sure that the colors from shot to shot will match.
The Waveform is generally considered the most robust exposure tool because it plots your image's luminance (brightness) -- and chrominance if using RGB mode -- to a graph but unlike the Histogram, it physically matches the screen from left-to-right.