One word Divergence The timeline started to diverge in 1945 Elvis Started his career in 1954. But to get to the real point you mistake the point of FNV its to bring back that "rat pack" 1940's to 50's feel. Elvis is the opposite of that just because you like there music dosen't mean they are fitting for the setting sure i like boston but i don't think peace of mind would be good on the radio.
Elvis was big. You know, there is a greaser faction with their head, The King. ^_^
And if Elvis isn't Vegas... The Rat Pack Vegas, by the way, would fit into the starting 60s.