Alan, I shall provide a few examples, one of the main ones being that I took hardly any care in trying to FIND examples, because they are just so plentiful, at least in how I view amount of vibrato.
The following I feel the vibrato is somewhat justified, but there is so much I can hardly tell what pitches she is singing, too wide, too often:
Same piece, different singer, same problem:
Some of that "tremolo" vibrato as I think of it. Too much. Sorry, but the soprano here hits some very high notes, and it might hurt your ears, so keep your finger on the volume for your speakers just in case. I had to take my headphones almost completely off... but I'm not concerned with the glass shattering highness, just the trembling vibrato, which I don't like much. The tenor has the same problem, I think, but as per the opera he is off-stage and hard to hear...
Here is my idea of just enough vibrato and not too much, the soprano has very very little, which I find perfect, and the alto is pushing it, a bit on the 'much' side, but still not at all to the point where it begins to sound bad:
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The three negative examples I don't think are too horrible, but still, my opinion says the vibrato is too much. What do the rest of you think?