I'm sorry to say that there is no real vibrato feature in Melodyne. But I have just tried this:
1) Take a long note that has no vibrato
2) Cut it up into approx. 10 short slices (note separation tool)
3) Select slices 1, 3, 5, 7, 9 and drag them slightly up (pitch tool + ALT)
4) Select slices 2, 4, 6, 8, 10 and drag them slightly down
5) If needed, tweak the pitch transitions
Voilá, here's your vibrato!
It took me about a minute to make an organ sing with vibrato. Whether or not the result sounds natural depends on your source material, though.
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