It could be that the Vulcans just don't have a rank of commander. Sub commander could be equivalent to SF commander, from there you go to Captain. There are plenty of times in TNG, and DS9 where LCDRs are referred to as "commander"...so the absence of this with T;\'Pol leads me to believe that either the Rank of commander doesn't exist for Vulcans, or the crew was attempting to point out that she was not part of Starfleet (as in, emphasizing her
Vulcan rank).