My idea: Vulcan was "conquered" by some warring dictator millennia ago (a la a super-successful Napoleon or Alexander), who formed the first Vulcan planetary government in their pre-Surak days (that broke apart before or during Surak's time).
Earth was never conquered by one figure, instead uniting under a democratic regime after the post-atomic horror.
Spock was thinking about extraterrestrial conquests, which Vulcan was spared. And when he spoke of collective memory, he was specifically referring to the logic-following Vulcan culture, not the one that predated it.
That's a very good answer. Maybe that might be combined with McCoy thinking that Vulcan was culturally "conquered" by Earth culture. And another possibility is that the hypothetical ancient Vulcan space empire was conquered in some space war thousands of years ago.
What really bothers me is Spock talking like the idea of a bunch of Vulcans - about 400 - all being killed all together at the same time is so unprecedented that it is considered to somehow be equal to the Vulcan state being conquered by another state.
How can the simultaneous deaths of 400 Vulcans be so unprecedented? If 400 Vulcans die in an average Earth minute then 24,000 Vulcans die in an average Earth hour and 576,000 Vulcans die in an average Earth day, and 210,384,000 Vulcans die in an average Earth year, and 21,038,400,000 Vulcans would die in an average Earth Century. If the average Vulcan died age 250 Earth years, then the total Vulcan population would be 52,596,000,000.
Star Trek (2009) says the total Vulcan population is 10,000,000,000.
If the total Vulcan population is 10,000,000,000, and the average Vulcan dies age 250 Earth years, then 40,000,000 Vulcans die in the average Earth year, and 109,514.03 Vulcans die in the average Earth day, and 4,563.0845 Vulcans die in the average Earth hour, and 76.0514 Vulcans die in the average Earth minute, and 1.2675 Vulcans die in the average Earth second.
So if exactly 400 Vulcans died in one earth second, Vulcans would be dying 315.5818 times as fast as normal for that one second. And if exactly 400 Vulcans died in one Earth minute, Vulcans would be dying 5.2596 times as fast as normal for that one minute. And if the exactly 400 Vulcans spread out their dying over a full Earth hour, they would be dying 0.08766 times as vast as the average rate of Vulcan dying. So it is certainly conceivable that random fluctuations in the rate that Vulcans die would make 400 Vulcans dying in one minute or even in one second be something that has happened before and is recorded the collective Vulcan memory.
There have been a lot of natural disasters in merely the 21st century on Earth in which a lot more than 400 people died in an hour, a minute, or a second. They include 70,000 killed in 2003 European Heat Wave, 87,587 killed in 2008 Sichuan Earthquake, 100,000 killed in the 2005 Kashmir Earthquake, 138,66 killed in Cyclone Nargis in Myamar in 2008, 160,000 dead in the 2010 Haitian Earthquake, and 280,000 dead in the 2004 Indian Ocean Tsunami.
Among deadly accidents in the 21st century were the sinkings of
Dong Fang Zhi Xing, 2015, 442 dead;
Salahuddin-2, 2002, 450 dead;
Senopatri Nusantara, 2006, 461 dead;
Princess of the Stars, 2008, 832 dead;
al-Salam Boccaccio 98, 2006, 1,012 dead;
Spice Island I, 2011, 1,573 dead; and
Le Joola, 2002, 1,863 dead.
So Vulcan science and technology and approach to risk management must be far more advanced than that of 21st century Earth if there haven't been many natural disasters and accidents within Vulcan memory in which over 400 Vulcans were killed at one time. And if the
Intrepid was the very first time in centuries or possibly millennia of Vulcan space exploration within Vulcan memory that an entire crew of 400 were killed, then Vulcan exploration procedures would seem to be in many ways superior to those of humans. And of course
Enterprise certainly indicated there were many times when Andorian space crews attempted to explode Vulcan space ships.
And I fail to see how hundreds of Vulcans being killed would seem unimaginable to Vulcans because Vulcan has never been conquered. That seems like a non sequitur.
And I always wondered why McCoy thought that sobriety would explain a planet being conquered. Maybe McCoy thought that sober people would never be reckless enough to fight back against invaders.
So a perfect explanation of the odd aspects of McCoy's and Spock's statements would be very desirable.