I was never bothered by the "Sub-commander" vs "Commander" situation; Vulcan system vs human system was enough for me. That being said, this explains things quite well,
Praetor. Thanks for the research
You're welcome. But clearly I missed a thing or two...
Offhand, I recall Captain Vanik of the Ti'Mur in "Breaking the Ice" and Captain Sopek of the Ni'Var in "Shadows of P'Jem." There might be others.
I'd seen, and forgotten, both of them!
There are five more listed on Memory Alpha
here along with a whole host of other Vulcan ranks. Whoops. And it has its own rank pin, so it's not just a title. Hurm. I guess my Romulan analogy falls short. I can't help but wonder if there are unseen Romulan captains hiding somewhere behind those Commanders we always saw, and Commander is more like Starfleet's Fleet Captain?
For the Vulcans at least, I suspect it's more a case that Commander is like Starfleet Captain Upper Half and Subcommander like Starfleet Captain Lower Half, for lack of a better term, and then the Vulcan Captain is still above that.
Oh well, if they can do away with emotions in a few thousand years, they can diverge in ranks too.