Just wondered where you'd heard that its nothing to do with it, as I've not really seen many versus details, not for lack of trying! They just dont exist.
I'm not saying it could be a storyline sequel or anything, Just set in the same world(a prequel in a sense). I was thinking along the lines of FF7, During play the world is futuristic and mechanical. At the end of the game when you finish it, You see Red looking over a grass overgrown Midgar. I'm thinking its a possibility for FF13vs. Technological advanced past(vs) with Pulse in FF13 being the results of an aged world
Same, to me it looks like Versus is going to be a prequel, set well before (the developed world of) Pulse is destroyed and the Fal'Cie build Cocoon.
The story itself may be nothing to do with that of FFXIII's, but the setting seems to be. The buildings and Tenjin's tower are very reminiscent of the buildings you seen in the Versus trailer.
very open to run around do missions i like it