The vibrato sound is created by plucking a string and changing the tension of the string by alternately pressing and releasing it between the tuning pin and the bridge pin with a finger of the other hand.
It has to occur on flat-notes and is the most audible in the two octaves above the middle C.
The effectivity of the vibrato depends also on the harp model and string type, and is certainly not working on wire strings.
The effect can be produced on one to four strings, even though it is the most successful by using the thumb. The range may vary, depending from the mentioned criteria from less than ¼ up to ½ tone.

The vibrato effect can be specified with different terms, for example with “like a vibrato”, “vibrated sound”, “comme un vibrato”, “make the string oscillate” or simply noting a “V” above the note. The dynamic goes from p to f.