Your best bet is probably routing one end of the SEA>LAS through LAX or SAN. PDX won't work as you'll get the minimum but lose enough distance to come up just short (looks like this is what you already figured). My brain is too tired to figure out if the bay area would work but that might be an option. LAX or SAN are usually pretty cheap. You could also look into a SEA>SoCal>Bay Area then grab a Southwest (WN) flight over to Vegas. I've used AS to get me to the west coast and then grabbed a cheap WN flight into a Vegas a few times as they often have a lot more flights in/out than AS.