Sorry im new with new vegas i only beat game with ncr ending. (just now lol)

The strip itself? Mr House had a defence system that intercepted any nukes heading towards the city. Thats the short story. Im sure others might be able to elaborate a bit more on exactly how he did it.

From the Wiki

House went to work on a secret plan to ensure the city would survive this apocalypse and that he would live to see the world after the war. He programmed multiple mainframes with satellite links meant to disable the vast majority of the Chinese missiles while in flight, then designed an array of high powered laser cannons, which he had installed on the roof of the Lucky 38, to deal with any missile his program had missed.

http://fallout.wikia.com/wiki/Robert_House

He wasn't able to stop all the nukes, some did make it past his system. CCNA has a link to Mr. House so that should fill you in on his story.

But all in all Las Vegas wasn't completely untouched and it suffered centuries of neglect.

thanks maybe ill play mr house next

Mr. House is our benevolent ruler of New Vegas. He saved Las Vegas by retrofitting the Lucky 38 into a mega SAM site basically, being able to remotely forcibly hack into 58 of the 77 atomic warheads and destroyed 9 others with a laser mounted cannon atop the Lucky 38. Unfortunately, due to the Mark II OS not arriving as intended, he had to make due with his buggy software, but the performance could have been worse given only 9 of 77 warheads got through on inferior software (I mean, seriously, who else could have done such a better job in an admittedly suboptimal than intended performance?). Then turning three brutish tribes into civilised people almost (but not quite) on par with Pre-War culture and mannerisms. As well as being the true saviour of Vegas, our beloved Mr. House is a truly charismatic leader and with looks to match, his moustache alone makes him http://www.youtube.com/watch?v=tRsoaDcyLDE.

We here at the Free Economic Zone of New Vegas hope you have enjoyed your tour and we look forward to you coming back with as much zeal in your heart as we carry!