In most episodes and movies, the idea that Vulcans don't lie is probably accepted as a lie. That is, everybody in-universe agrees that Vulcans are
weird and deny themselves things for no good reason, so it's plausible they would deny themselves the pleasures and benefits of lying, too. But the characters would always have a lingering suspicion about that.
However, in "Data's Day", our android hero Data, to whom adjectives such as "literal-minded" and "objective" are easily applied, seems to accept the "Vulcans don't lie" claim axiomatically, in a situation where a Vulcan lying for sinister reasons would be the most logical explanation. This in an episode where Data is offering the audience his innermost thoughts, through letting us read his correspondence. Generally, Data seems to know a lot about verbal deception and manipulation, and has practiced it himself several times by the time of "Data's Day". Is he deceiving the audience
when seemingly claiming that he dismisses all suspicion of a Vulcan character on grounds of the she-doesn't-lie axiom? Or does he perhaps have a blind spot with Vulcans, a fellow species from the "we don't have emotions" group of denialists?
I guess Spock is always
lying by insisting that he doesn't emote. All Vulcans constantly lie in that sense. Yes, they sometimes "admit" that they are controlling their internally raging emotions, but that's a whopper in itself: emotions appear to affect all of their behavior, even if it never amounts to emoting.