Well, her voice started "shaking" as she sung sometime during LOVEppears. Of course, that doesn't REALLY count as a vibrato if a vibrato in Ayu standards goes like ThISisISisISis.
Ayu held her notes quite still during I am... though. Like, a good example would be A song is born. She had a lot of places where she could have just used a vibrato but she held the note instead in the chorus. She did do a little bit of vibrato but I don't think it was until Voyage that it became obvious.
Voyage's vibrato is quite famous I think. At the time, people made fun of it because it was quite new and all.
Nowadays people are used to it though.
Edit: And it's quite strange for people to say she started using her vibrato in 2007 because Ayu did a lot of vibrato in her lives during 2002-4 ish when she performed Voyage and No way to say and so on.