Ok new vegas is set in the future right? So if project purity worked, wouldnt that mean that there would be grass and a bunch of other good stuff?
Project Purity would have nothing to do with the west coast. If you played the first 2 games, water was not an issue (except for Fallout, where you find the water chip, but that was just Vault 13 and outside settlements had access to clean water). In Fallout 2, many settlements had water, power, and the major ones (like Vault City and NCR) had green grass and green leaves on the trees, all years before the events of Fallout 3. So, the west coast, including Vegas (that is, if the situation was decent there) would have access to all those things like water, grass, and trees, etc., regardless of the events that happened in Fallout 3.
Of course, who knows what New Vegas is like. It could be riddled with crime, like New Reno, but still New Reno had its luxuries. We won't know until the game is released (but speculation is always fun
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What I think would make a great game is if it was set in Central USA. With that, there will be a chance to explain why the east coast didn't fair as well as the west coast did 200 years after the war. West coast technologies and ideas couldn't travel across the Rocky Mountains, rivers, Great Plains, Appalachian Mountains, etc., because of what reasons? Here's something else to think about: according to Fallout 2, all vaults were (supposedly, if there was no error) given at least 2 G.E.C.K.'s, so what happened on the east? You would think that making sure the east coasts vaults were in tip top working order having all their supplies to be top priority over the west coast (because, you know, the government is located there), so what happened? Why couldn't they get their act together in 200 years?
Of course, that previous paragraph would be good for Fallout 4, since New Vegas is really not a sequel to the main games.