Also, Alice and Emily have fantastic vibratos as well.
Michael Rupert's is very interesting as well.
If you could catch Donna Vivino in "Martin Short: Fame Becomes Me" while Nicole was out, she had such a good vibrato. Sounded just beautiful. It was fast without being too shaky or annoying, but the way I saw it, her vibratos have varying speed with the style of song that's sung, much like Liz Larsen or Lea Salonga or people like them.
"How could she just suddenly, completely disappear into thin water?" - The Little Mermaid