Didn't Grahman say that it was Death Valley? Filled with storms and that the incident that happened there means that the NCR reinforcements are weaker? If so then it is - self-evidently - in the Death Valley National Park which, like Grahman says, would hinder NCR reinforcements as it lies directly in-between Vegas and Shady Sands (if we take Sand's to be slightly south-east of Mt. Whitney, Vault 13's approx location).
EDIT: Would also make more sense than the Grand Canyon which is in the complete opposite direct from the Canyon Wreckage, which incidently would also lead to Death Valley.
EDIT 2: Graham also mentions Route's 127 & 88 which are also close to Death Valley in California.
There is essentially no doubt that it will be in or right next to Death Valley, as it was stated explicitly in Honest Hearts. I wonder if the road we'll be travelling is the entire way to Death Valley? It would be awesome if the expedition was a week long if you bothered sleeping, and as the map isn't Point Lookout-like and is Point A to B, it wouldn't be unreasonable.