The cap is much paler than any Blue-headed I've ever seen and is not as crisply defined along the malar as Blue-headed. The lore is noticeably darker than the rest of the cap.
There is markedly reduced yellow along the flanks; it essentially lacks yellow along the flanks. The only area where there's clearly yellowish wash is about the vent.
The bird then came out into the sun and the slightly olive tinged gray back really jumped out, it's not at all like the green back of a normal Blue-headed.
The back color extends up over the back of the head which apparently is cited as a pro-Cassin's feature in an article I can't find.
The bird popped up twice more for us, usually pretty high in amongst the warblers.
Unfortunately Blue-headed isn't a species that I've had a ton of luck photographing in the past so I don't have a lot of comparison shots. Hopefully I can find some of the articles that talk about separating the two. I was kind of surprised to see that Cassin's Vireo is widespread in the West east of the Rockies; I had always considered it a Pacific coast bird. They do winter in Texas so it's surprising there aren't more seen in the East though findability and confirmability issues probably play a role in this.
I haven't edited the pics at all except for cropping them.