They play identical until you use the dark force and the Dark Ability which is unique to VS2 in a funky way.
VS Dark Force grants you the Dark Ability unique to each character, that's about it, nothing else about Dark Force other than being bale to get hit without cutting out the Dark Force time.
VS2 Dark Force is a state of damage in which you can inflict damage without red recoup, in other words each hit drains life dry without a chance for regeneration. Also while in Dark force all your supers have level 3 access, as in stronger supers or ultimate version of them. The Dark Force ability can be activated at any time, no need for Dark Force but it goes away if you're hit so it's key to use it wisely.
2 very different systems and a bit of a difference in the roster.
AJ, you hurted my feelings in VS 'cause I know I'm stronger than our last showdown. Once I get back in the thick of things with VS I'll haunt you for a rematch and show you the true extent of my darkness...the salt river never comes back...