D.C Fontana's First Draft script for "Journey to Babel" (dated August 22, 1967) has a comment that provides some perspective:
KIRK
There are other Vulcans aboard.
SAREK
I am afraid you will encounter
some difficulty, doctor. You
will note my blood type is T-
negative...somewhat rare even
for Vulcans. Your chances of
finding it in my four aids are...
SPOCK
(abruptly)
I have T-negative blood.
So as originally written, the "other Vulcans on board" comment was meant to pertain to the Vulcan's in Sarek's ambassadorial party.
KIRK
There are other Vulcans aboard.
SAREK
I am afraid you will encounter
some difficulty, doctor. You
will note my blood type is T-
negative...somewhat rare even
for Vulcans. Your chances of
finding it in my four aids are...
SPOCK
(abruptly)
I have T-negative blood.
So as originally written, the "other Vulcans on board" comment was meant to pertain to the Vulcan's in Sarek's ambassadorial party.
In Journey to Babel during a discussion in the sick bay, Kirk states "there are other Vulcans on board". We don't know if he refers to other traveling dignitaries or members of the crew, but I assume he refers to crew, given that we do not see any other Vulcans with Sarek and Amanda.
BTW - It would have been the perfect time to introduce and utilize Dr. M'Benga.