Corrected entry: In the episode Flashback, it seems that the producers didn't watch Star Trek VI. In the scene where Mr. Tuvok explains to Captain Janeway about the explosion of the Klingon moon, he says that Captain Kirk and Dr. McCoy were arrested two days after the explosion, when it actually happened over two months after the explosion.
Corrected entry: In the episode Flashback, during the first mind meld between Captain Janeway and Mr. Tuvok, when they first show up at the bridge of the Excelsior, is mentioned a rescue mission of Captain Kirk and Dr. McCoy, who were arrested due to the assassination of the Klingon Chancellor. However, on Star Trek VI - The Undiscovered Country, there is no mention of such mission mainly because the only ship that was nearby Klingon Space was the Enterprise.
Correction: Captain Sulu was operating against orders and without knowledge of StarFleet Command, why would it be mentioned in the first place?
Correction: This could be a simple character mistake. The events of "The undiscovered country" take place in 2293 whilst this episode takes place in 2373 & Whilst we have the videos or dvds & can watch the films & series whenever we want, at the time Tuvok only has his memory to go off & is under stress from the memory virus. It's likely that later on he would have corrected himself to the captain.
Indeed, Tuvok remembers Valtaine dying on Excelsior's bridge, but he was shown alive and well at the end of ST:VI. In fact, Tuvok remembers himself being on Excelsior, when he wasn't shown in ST:VI.