My take is that the Intrepid crew was mostly Vulcan, just as the Enterprise crew was mostly human. Kirk and McCoy ares the ones who comment that the Intrepid is manned by Vulcans, as humans they're allowed to be imprecise. Spock only mentions the Vulcan crew in reference to their "psychic death scream".
C.E. Evans said:
I always just chalked it up that some ships are crewed by whoever is on hand when they launch on various missions. If both the Intrepid and the T'Kumbra were at Vulcan at the time of their respective deployments, it doesn't seem too unreasonable that they might have all-Vulcan crews. I think ships with single-species crews are rare among the fleet, but they do happen now and then.
Would the ships be totally devoid of a crew until they're assigned a mission?