I have to check dictionaries now. Vibrato is definitely defined by pitch varying, usually in a regular pattern. It happens as a natural part of voice production. If there's any variation in loudness, its quite small and subtle.
It USED to be that 'vibrato' was used to describe exaggerated or ugly 'wobbling' like Maria Calas' high notes as her voice worsened with age and strain. Early 20th century writers would decry 'vibrato,' but the same writers would hold up Caruso as the model of perfect singing...and Caruso's (healthy) vibrato was quite pronounced.
Now that I look, 'tremolo' is trickier in definition. It IS a synonym for vibrato, but also can mean quickly repeated or 'pulsed' notes as well.
I was using tremolo to describe the ugly quaver of some players/singers. Madonna, when they engineers haven't had a chance to clean up her tracks. Andrea Bocelli, who I find downright goatish sounding.
Muggsy's sound on these tracks is almost a growling effect. Natty Dominique seems to be cultivating that quality on purpose. Bix certainly starts his solo on the Wolverine's 'Royal Garden Blues' with a 'tremulous' high note. Perhaps not on purpose. In a year or so, Bix is fluid enough to to shape and bend tones at will. Something most of his followers don't seem to have picked up on at all.