Vibrato reads the signal from a delay line whose length is being modulated by an LFO. As the read head moves closer to the write head the audio arrives slightly early, which raises its pitch, and as it moves away the pitch falls.
Rate
Rate covers 0.1 to 12 Hz on a log scale, so most of the travel sits in the musical range under 8 Hz.
Slow settings under 1 Hz drift rather than wobble, which is closer to tape flutter than to a performance. 4 to 6 Hz is where instrumental vibrato usually lands.
Depth
Depth is how far the delay swings, from 0 to 8 ms. The resulting pitch change depends on rate as well as depth, because pitch follows how quickly the delay is changing rather than how long it is.
At 1 ms and 5 Hz the shift is subtle enough to pass as playing. Past 4 ms it is an obvious seasick effect, which is the point on a synth line and a problem on a vocal.
Stereo phase
Stereo phase offsets the modulation between channels by up to 180 degrees.
At 0 both sides bend together and the pitch moves as one. At 180 they bend in opposition, which widens the image because the two channels are never at the same pitch at the same moment.
Mix
Mix blends the modulated path against the original, from 0 to 100%.
Vibrato is normally used fully wet. Bringing the mix down to around 50% puts the original pitch back alongside the moving one, and the beating between them turns the effect into a chorus.
Example: a synth lead
Rate 5.5 Hz, depth 1.5 ms, stereo phase 30 degrees, mix 100%.
The line gains the movement of a played instrument, and the small phase offset keeps it from sounding like one static source.