In the 50's, Elvis was still getting his career going in Memphis. Other than a concert or two, he may have not performed in Vegas much in the 50's.
It's not the 50's anymore.
They have an entire school devoted to Elvis impersonation, in game... That alone implies the passage of time, and that by the point in the timeline that we are playing Fallout, Elvis has risen through the charts to become the King, then raised his cholesterol through peanut butter sandwiches till he died. There were impersonators before he died, but not until after the 50s stage of his career.