i think when vulcans say they “cannot lie” they mean it in the same sense as when I say I “cannot dance”
physically and mentally capable of going through the motions
just very very very very bad at it
the vulcan lack of lying skill is in harmony, not conflict, with the vulcan poker face
easier to keep a poker face when you have no deception to hide. easier to refrain from deception when that’s the only way to keep your precious poker face bc your face gives away any lies instantly
the poker face and the brutal honesty are in a symbiotic relationship. either one without the other would crumble