Question: In *Star Trek: Generations*, the actor Tim Russ plays an unnamed bridge officer (probably tactical/operations, as he's monitoring the ship's hull and systems integrity) on the Enterprise-B in the prelude. No pointy ears or eyebrows, so I must assume this character is not Vulcan and therefore not the Tuvok character that this same actor portrays in Star Trek: Voyager.

Was there ever an in-universe explanation given for this actor's appearance as someone other than Tuvok? Obviously any such explanation would be a retcon, as Generations was released a year before Voyager and so Tuvok's character didn't exist. But was there even an attempt, or is the role just dismissable as a human who happens to look similar to a notable Vulcan?

Answer: 
I also feel myself bugged by his appearance because it bring us automatically to Tuvok (Voyager). But there is not any relationship. First because the guy at Ent-B was an human and not a vulcan; second because the Ent-B was commissioned almost 80 years before Voyager. So...
Although this isn't an official piece, my favourite personal theory for Tim Russ' appearance elsewhere in Star Trek is because he's an undercover agent (he had previously infiltrated the Marquis and had been brainwashed during said stint in the official VOY diaglogue). Make-up and physical changes aren't uncommon in Star Trek - such as when Janeway becomes borgified.

So it might well be, as a Vulcan with a 200 year long lifespan, that he could very well appear both on Enterprise-B, then later a mercenary on the Enterprise (especially as this might have been during his undercover stint for the Marquis - we never actually see him die, he could have just cleverly beamed out back to his ship whilst screaming to fool Picard) and then again as the Klingon mercenary on DS9 (perhaps trying to expose any Dominion connections?).

This might explain, after all, why Tuvok is such a badass.

Who knows? But it's the theory I like to entertain. Makes it more interesting.