Vector synthesis is a method of creating sound that blends two or more oscillators and allows that blend to change over time. It is typically done with two or four oscillators and synthesizers that use this synthesis method typically have some sort of joystick or X-Y control surface.
It was popularized by the Sequential Circuits Prophet VS and followed later by the Korg Wavestation and Yamaha SY22/35 synthesizers.
While it is more common for this synthesis method to be used with sampled waveforms, it can be used with any type of oscillator.
In addition, the Korg OASYS and Kronos synthesizers have vector capabilities.