I don't think a slightly trilled r is to be confused with the speed or variation of pitch or oscillation in a vibrato, Alex - but certainly a faster vibrato and that clipped pronunciation were both more frequently encountered in the past, whereas today the slow wobble is much more prevalent and problematic. Some people hear the fast vibrato of the earliest recorded tenors such as Bonci as a kind of bleat - later, Corelli had that problem and cured himself of it, whereas to my ears a modern tenor like Calleja remains afflicted with it - and similarly Conchita Supervia's mezzo is often described as a "rattle". There is certainly an optimum speed and variation but also a certain tolerance within aesthetically determined parameters - and the only singer I know who could alter his vibrato tastefully at will was Lawrence Tibbett; he often speeded it up for dramatic effect. I find certain Bayreuth bass-baritones, current and of the recent past, unlistenable, murdering the role of Wotan, they have such a slow and prominent pulse.