This was the first eastern Bell's Vireo recorded in Santa Clara County. See the excellent May 2019 article by Kenn Kaufman and Brian Small on the subspecies of Bell's in "Birdwatching". Along with ID tips, there's a discussion of the likely split into two species, the southwestern (California/Arizona) and eastern/midwestern Bell's. A proposal of this split in 2017, supported by substantial DNA evidence, failed by the narrowest of margins, an equally divided vote of the Checklist Committee, accompanied by a recommendation of further research on the birds in the overlap area.
See https://www.birdwatchingdaily.com/birds/kenn-kaufmans-id-tips/tips-for-identifying-bells-vireo/