Not to be outdone by an early Famitsu reveal, Platinum Games updated their site with Vanquish story info and screenshots.
The reason why you’re in a transforming suit is because the United States and Russia are at war over energy resources. Vanquish takes place in a space station occupied by Russian forces. It was originally built to transfer solar energy, but has been converted to a solar powered laser that wiped out San Francisco. Sam has to reclaim the space station (from robots) before it’s used to blast New York.
The English description from Platinum Games also notes Vanquish is due in stores this winter, which suggests we’ll get this game around the same time as Japan.