Originally Posted by
Spatvark
Ivalice is not a world. This needs to be stressed, something people seem to miss a lot. In FFT and Vagrant Story, Ivalice is a kingdom. Meanwhile, in FF12 it's a region (like say, the Baltics), which includes Valendia (where Vagrant Story is set), Ordalia (the nation that had been at war with Ivalice before the beginning of FFT, now a continent, so it probably absorbed what had been the kingdom of Ivalice to the west of it, whilst Ivalice's name passed to the region), and Kerwon (which is entirely new).
FFT is set in Ivalice. FFT:A is set on a southern peninsula of FFT's Ivalice, by the looks of it to me, comparing the FFT and FFT:A maps. FF12 is set in the Ivalice region; the map I'm looking at suggests it's just east of the edge of the FFT map. VS is set in a small region of the northern continent of the FF12 map, though no map is ever seen in VS to offer visual evidence of this.
Basically, it could easily be the same world in all four games; the only thing that really goes against it is the lack of Bangaa, Nu Mou and Viera in FFT, but even that is far from definitive. All kinds of reasons could exist for that to have been so. So yeah, might, might not.
Images are FF12 Ivalice, FFT:A Ivalice, and FFT Ivalice, from left to right.