But I sort of thought of her as a 'guest star', as opposed to a Mary Sue.
Here's the thing: In '60s and '70s TV, it was commonplace for TV series to take a semi-anthology approach where the guest stars of the week were the centers of the story and the main cast were more like supporting players in their journeys. The Western
Wagon Train was a well-known example -- almost all its episodes were named "The [Guest Star] Story" -- and when Roddenberry pitched
Star Trek to network execs as "
Wagon Train to the stars," part of what he meant was that he intended to use that same semi-anthology approach (not just "Western in space," because half the shows on the air back then were Westerns, so that's not why he used that particular one as his example). You can see that in first-season episodes like "Mudd's Women" or "Charlie X." That subsided later when Spock became the breakout star and all the episodes ended up centering on him, Kirk, and McCoy.
So there's nothing intrinsically wrong with a story where the featured guest star is the central driving character in the narrative. That was normal enough in the series fiction of the era, and thus normal enough for Trek fan and tie-in authors to emulate. And it was natural enough to make those guest stars strong and impressive women, in order to compensate for the dearth of female lead characters in the show. The story that introduced the "Ensign Mary Sue" character was a parody of those fan fiction stories that did it
badly, that were badly written author-insertion fantasies where the guest character was described as better than the main characters but never actually did anything to demonstrate that worth. The problem is that it's no longer common for fiction to center on guest stars instead of the leads, so people have gotten the mistaken idea that
any guest character who's the center of attention is a Mary Sue, rather than just the failed attempts at that otherwise valid character type.
The featured guest in
Vulcan! is definitely a Mary Sue, however, because the leads are written out of character in order to make her seem more impressive than she is. Spock is irrationally, blindly convinced of a theory that turns out to be wrong just so that the guest star can prove she's smarter than he is. And pretty much everyone on the ship is more forgiving of her rank bigotry toward Vulcans than they realistically would be, because they find her so charming and impressive in every other way.