I would try to change your 4/6 SEA-LAS nonstop to add a couple stops on your way down to Vegas.
SEA-SFO-LAX-LAS
SEA-BOI-LAX-LAS
SEA-PDX-SFO-LAS
etc.
That should get you the extra 484 miles you need.
On the AS website, when you go to change your 4/6 flight, it should spit out some additional flight options with stops and show the difference in price, which may cost a bit more than your current ticket.