Is there a way back to vermilion wastelands within the postgame?

[Chocos Ramabotti]

I still have 2 chests left there, but there seems to be no way back, teleporting doesn't work, am I locked out of completion?

[Merim]

Depends on the location, your only bet is through DLC at the very end of the game. But you are limited to some areas, if its outside of the limited areas, then its impossible and you have to play new game.