Perhaps we assume that Vulcans can't lie based on the Romulan Commander's and Spock's conversation in The Enterprise Incident:
"There's a well-known saying – or is it a myth – that Vulcans are incapable of lying."
"It is no myth."
- Romulan Commander and Spock, as she interrogates him
Of course, we now know that Spock was lying at the time...