Interestingly enough, Spock has a "get out of jail free" card where he can make a case for NOT lying to the Romulan commander about lying.
She says "There is an old saying, or is it a myth, that Vulcans cannot lie...?"
Spock replies: "It is no myth..."
Technically speaking, he could make the case that he was saying: "It is not a myth that there is an old saying that 'Vulcans cannot lie'...", which would be literally true...just not answering her REAL question and letting her assume that he did.
By the way, another case of Spock lying is in "The Tholian Web" when he joins Bones in denying that they'd listened to Kirk's message tape.
Frankly, it is the case, IMO, that most of the "Vulcans are pacifists/vegetarians'[insert behavior preference x]" carries an implicit caveat of "generally speaking" and "by choice".