I think you're on to something. Michael Burnham was commissioned into Starfleet without attending the Academy, having gotten her education of Vulcan. Many Vulcans in Starfleet might've transferred in. That still leaves the question of how Spock, widely-recognized as a super-genius even by Vulcan standards, didn't graduate at the head of his class, but that could be down to some specific aspect of how Starfleet calculates GPA that Spock didn't find it logical to indulge in merely for the sake of having a higher numerical ranking (say, he would've had to take an additional course to push up his average grade that he expected to have no use for in his career, and believed there were better uses of his time and effort).
If Spock's goal was to graduate No1 at SFA, then he would have. "being the first Vulcan to attend SFA"
was more notable then he graduated at the top of his class?
I can see Vulcans being given SF commissions based on Vulcan space service experience. Not certain about a ship entirely Vulcans, short of special circumstances or influence.