This is a tough one, but reviewing this article: https://www.surfbirds.com/Features/Solitary_Vireos/identification.html I favor Blue-headed Vireo for your bird.
I spent 30 minutes comparing my photos and this article. The birds I see around Teotitlan, western Oaxaca has much greyer head than those of Tuxtepec, where Blue-headed V. is common. This is why I guessed Cassin's. But I understand Blue-headed is variable...This is a tough one, but reviewing this article: https://www.surfbirds.com/Features/Solitary_Vireos/identification.html I favor Blue-headed Vireo for your bird.
You need a "better" photo ? I think the photo is perfect, isn't it ? I didn't find a single one better than this online. I've plenty of photos of this individual, which details you want to see to help ?If I had to guess I would say Blue-headed, but without better photos it is best left Cassin's/Blue-headed Vireo.
Thank you Niels. Let me add a few photos.I agree this is as good as any single image can be. However, coloration of the underside is very strong. I would like to see how the rear underside looks in an image more from below and I would like to see the rump/back in a rear view image if available.
thank you for your input. I'm not sure if my bird will be "certified" as Cassin's one day, but for sure I learned a lot ; I've seen an obvious Plumbeous Vireo in the same area a few days ago, a nice field of research I guess...I have reviewed the two photos again against the photos included in above paper. I am now leaning Cassin's. Compare especially the first couple of images in the paper (the one from Vancouver Island) and the notes below. I feel the contrast between throat and side of head as well as contrast of lores in the OP images are very similar to what you see in the bird from Vancouver island and less than what you see in those labeled as true BhVi.