@david7 - this is a great question, and a big thanks to @KeroroRinChou for sharing that awesome YT vid excerpt of Todd Parrott talking about vibrato, and I think he has one of the sweetest vibrato’s out there.
Notice that he said “It didn’t sounds like this at the beginning!” Like most things with harmonica and life, we use way too much tension when we’re first learning, and then gradually we learn to relax into it more and more.
Learning how to do the “coughing” technique in a softer more relaxed manner like @KeroroRinChou called it “gulping” - I think that’s probably a helpful way of thinking about it.
I think there are basically 2 ways you can approach getting faster and more controlled vibratos.
I asked my vocal teacher about vibrato, and he told me it’s 5-6 vibrations per second. So I started putting a metronome on 60 BPM, and worked on draw notes doing 2 “coughs” (or “gulps”) per second, and then got it up to 3, and then 4, and then eventually 5, and now that I’ve been doing it for a couple years I can even get 6. While doing this, you can experiment with putting your tongue in various positions to vary the depth of the vibrato.
But the other method, which I think is superior because it puts such great demand on other aspects of technique - especially breathing - is what Joe Filisko calls the “Big Tone Train” which he talks about here:
I’m guessing that @davidkachalon thinks that playing the train is the best way to develop vibrato speed and control?