In the first episode of Star Trek Voyager it is revealed that the Vulcan character Tuvok was sent to be a double agent for the Federation in the Maquis. There are conversations in Next Gen that say Vulcans can't lie, I believe in the episode where Spock's dad shows up.
Is there an in universe answer to why they would send someone who cant lie to be a double agent and how he got away with it (except for that one guy)?
Vulcan can lie, they typically don't find it as the most logical course of action. Humans lie much more often in comparison, making the lack of lying noticeable. In every series we see Vulcans lying, lying by omission, committing subterfuge, sabotage, even straight up spy work, complete with murder. Spock lies when commanded, and on many missions. One time he was going to lie to McCoy but realized Vulcan neck pinching the bastard to sleep was better.
Tuvok himself even addresses this, during a conversation with Chakotay in s1e10 "State of Flux":
(Chakotay goes straight to Tuvok, who is reading at his corner table.)
CHAKOTAY: Can I ask you to be honest with me, Lieutenant?
TUVOK: As a Vulcan, I am at all times honest, Commander.
CHAKOTAY: That's not exactly true. You lied to me when you passed yourself off as a Maquis to get on my crew.
TUVOK: I was honest to my own convictions within the defined parameters of my mission.
CHAKOTAY: You damned Vulcans and your defined parameters. That's easy for you.
TUVOK: On the contrary. The demands on a Vulcan's character are extraordinarily difficult. Do not mistake composure for ease. How may I be honest with you today?
CHAKOTAY: I'd just like to know, from someone else who pulled the wool over my eyes, was I particularly naive? Was I not paying enough attention? What the hell was it that let all you spies get by me?
TUVOK: Like all humans, you depend on feelings and instincts to guide you, and they invariably let you down. But particularly naive? No, Commander, and I've always considered your attention span to be adequate.
CHAKOTAY: Did you ever see anything about Seska that make you suspicious?
TUVOK: No. She quite expertly pulled the wool over my eyes as well.
CHAKOTAY: Well, that makes me feel a little better. Thanks.
TUVOK: Curious.
CHAKOTAY: What?
TUVOK: That my failure, added to your own, should improve your feelings.
CHAKOTAY: Misery loves company, Tuvok.
or Neelix. He lies only when required, by logic and or duty. Look at the episode where he steals space folding technology, against the Prime Directive, without telling Janeway.
