VCO stands for voltage control oscillator.
Let me make a suggestion first. Down load the VX3 manual from the Whites site. It will be a PDF. Now you can read about all the functions. But the PDF also has the advantage that it has a search feature. You can type in VCO and it will take you right to the page. You can also use the on board help on the detector.
If VCO audio is disabled then the all-metal audio response will be a fixed tone, and an increasing signal level creates an
increase in its loudness: In other words, as the search coil is passed over a target, the all metal audio will begin at a low volume and progressively rise in loudness as the target signal gets stronger, peaking when the search coil is directly over the target.
If VCO audio is enabled then an increasing signal level creates a rising audio tone or pitch: In other words, as the search coil is passed over a target, the all metal audio will begin at a low tone and progressively rise in pitch as the target signal gets stronger. The highest pitch is achieved when the search coil is directly over the target (strongest
target signal) and then the audio falls in pitch as the coil moves off the target. As with non-VCO audio, loudness also
rises and falls. ROB