i was thinking quite early in her career..

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. :P

Voyage's vibrato is quite famous I think. At the time, people made fun of it because it was quite new and all. :P 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.

^and forgiveness is like... the over-vibrato ballad of death xD

Yes, there is mild vibrato in LOVEppears and it becomes a little more prominent in Duty. It's definitely more audible in her I am... album. However, you can certainly hear it stick out in the H single (I can't remember any vibrato in Free & Easy...). So, I'd say 2002 is where it REALLY started to become quite... "obvious," for lack of a better word. :P

And yes, as everyone else said: Voyage. :P

Ayumi is certainly capable of a natural sounding vibrato. I can hear it in her voice. ;) But I guess she just likes to (over)exaggerate it. lol XD

it's very nice if you use it correctly, and it's very hard to master. it took me a while, but i've gotten there.