I can't remember if I heard this bird vocalize or not, but I did classify it as a Cassin's. I saw it on Lone Tree Rd near Hollister CA which is Debbie Shearwater's dry land patch. I was scheduled for her pelagic the next day out of Monterey and had some time to kill in the afternoon. So I looked in eBird for nearby spots and found that Debbie had frequented this particular location many times. What a great road. It starts in agricultural land and winds up into some really nice pasture land in the hills and ends up in some nice oak forests. I checked eBird and the other two similar vireos have never been seen in this spot and would be more rare: Plumbeous and Blue-headed.