In NW the Commander mode allows you to control a regiment of your own bots - kind of like in single player - along side other players doing the same thing, versus other players doing the same thing. Meanwhile the FaS 'Captain' mode allows you to use cash to purchase mercs which you can control to take on other players and their mercs. Both of these are very different to VI which, as far as I know, does not give you control over your own troops.
The mode I brought up gets mentioned at the end of this video (as well as Invasion) which was made a few weeks prior to release at which time 'Warlords' was on the official list of modes to come.
edit: sorry, I can't get the links to activate for some reason.
When this game was still in pre-release they listed 'Warlords' as a multi-player mode. I was certain that this was going to be a commander/captain style mode because... why would any post-warband release NOT include such a mode?
But now that VC has been released Warlords is no longer on the list. Medieval-style commander battles was mostly why I was planning on buying this game so I'm very disappointed and probably won't buy VC until such a mode is released since, well, I'm really not interested in being Thor.