To all the people asking why not Mr House because all he cares about is Vegas. Thats not going to save Mankind entirely, just for Vegas. Which is nuts. Than all mankind is going to be living in is gambling.
Nope, if House comes to Vegas, he can rebuild it to it's Pre-War state, and soon after, reignite technologic reproduction of Pre-War tech, which could be sent to NCR, NCR could use that developed tech to further stabalize and civilize what they own. House would run Vegas as a business, which is more or less all Vegas ever was. House controlling Vegas and rebuilding the his slice of the world would produce technologic exports to NCR and any other potential uprising nations of stability (Not the Legion since they are ludites).
Also, who cares if House only saves just Vegas? He still saves Man to a larger extent than the US Government ever did, and he is establishing his own business zone. You need a business mindset to truly understand how things would pull in the long run.