Or we had a few Vulcans (like Spock's half-brother and himself) that disagreed with the conservative attitudes of the majority of Vulcans and pursued a career in Starfleet instead. Maybe there were just 400 of them (and not 430).
But I think T'Girl
was really on to something. Spock did not refer to "Vulcans" but to "Vulcan's race".
According to Spock in "Balance of Terror" there had been an aggressive colonizing period in Vulcan's past, thus it stands to reason that we had pre-Surak Vulcan colonies on other planets (e.g. the Rigelians mentioned in "Journey to Babel"?).
There might have been an armed conflict between Earth and one of those Vulcan colonies ("Vulcanian expedition") in the pre-TOS time that would explain quite a lot of the anti-Vulcan resentment we have come to see in early TOS.