I wouldn't think that a single mind would have much impact if there were millions or hundreds of thousands or even just thousands of Vulcans.
From "Mirror, Mirror":
SPOCK: One man cannot summon the future.
KIRK: But one man can change the present.
Perhaps those thousands or millions of Vulcans felt a brief moment of clarity, standing up out of their bathtubs and shouting, "Eureka" (which in Vulcan means "This bath water is too hot!") After that, they directed their energies towards finding out what had triggered it in everyone
I'm not suggesting the change happened instantly. Who knows how long it took the Vulcans to establish their philosophy of total logic? And we don't know how long they'd been practicing it when TOS rolled around.
And of course, there's the whole but-that-would-be-circular argument.
Reflexive causality is perfectly legit. The Enterprise
was part of the history they were sent to observe in "Assignment Earth."
I've thought that a neat twist to the last BACK TO THE FUTURE movie would have been to take 1955 Doc back to 1885 to rescue 1985 Doc. (Which would be a pair 'o' Docs.) After the mission was over, they'd return 1955 Doc, first, but accidentally arrive a week too early at Doc's favorite red letter date in the history of science. The "BOOM" of the Outatime arriving would startle Doc, who was inside the house standing on a toilet while hanging a clock... (They'd realize they were too early and jump forward a week to drop off 1955 Doc.)