Vibrato is an important part of the blues harp sound, there are two ways to get a vibrato effect and you really need to know how to do both because they are used differently in various playing situations.
You can get both a wah effect and vibrato by using the hand, to do this you simply make a good seal while cupping your harp with both hands then release one hand to let the sound out. You can vary the speed to suite the music.
This is where you use a laughing type action to block and release air from your throat, this can be used on both draw and blow, single notes and chords. It can also be used to semi bend a draw note to give a warbling effect. One important thing to note is that the vibrato should be completely controllable, from fast to slow, loud to quiet. A deep vibrato sound will also involve control from the diaphragm. This technique probably won’t come straight away and will require a bit of perseverance but it is worth it because it’s an important part of the blues sound.