I pretty much assumed that he was the first 'Human/Vulcan Hybrid' in Star Fleet.It is called Fanon. Like Spock being the first Vulcan in Starfleet. Oops...
But technically, T'Pol (as far as we know) only ever served on board a Pre-Federation Earth spaceship and Burnham is a Human who lived with Vulcans.
So nothing so far, (that I can remember) has dissuaded me from believing that Spock wasn't the first Vulcan to serve on an actual Federation Star Fleet starship.

Addendum:
I forgot about the USS INTREPID (NCC-1631)...
Completely manned by Vulcans.
Though that set of circumstances could have evolved after Spock joined Star Fleet and went through the academy.
