When I first started to play the theremin, I tried very hard to control the width of my vibrato -- perhaps in a self-conscious effort to avoid an overly-wide vibrato.
I would have made faster progress if I had focused on placing the center of the vibrato precisely on the target note without over emphasizing the width.
In controlling the width, I tended to put the center of the vibrato below the target note. This seems to be a tendency among thereminists in general.
The ear tends to latch on to the top of the vibrato and to tune to the top of the cycle places everything else flat.
Pamelia Kurstin is acknowledged to have a particularly keen sense of pitch and she places the top of the vibrato clearly above the target pitch. Her vibrato is, in reality, quite wide however it never occurs as "overdone".
Speaking of Pamelia Kurstin, she remembers to keep her vibrato active on the tail of notes.
When starting out I had a tendency to make my vibrato slower and narrower on the note tails. However, my practice recordings revealed that to keep the vibrato going on the tails is much more effective. Again, Pamelia Kurstin kind of shows the way on this... in fact, if my ears are not fooling me, she may widen her vibrato on the note tails.
I still like my note heads to have minimal vibrato so that there is "somewhere to go" as the note continues. However, I tend to like the sound when the tail continues with vibrato.
Thus, I suggest that while one may be aware of one's vibrato width, one need not fear it! The placement of the center of the vibrato is more critical than the width.
[i]-- Kevin[/i]