The shortest trans-Pacific commercial flights are Tokyo (TYO, includes Haneda & Narita airports) to Seattle (SEA) or Vancouver (YVR), about nine hours. So, you could fly BKK-TYO-SEA/YVR-LAS.
Checking TYO-SEA, it's flown non-stop by ANA (NH), Japan Airlines (JL) and Delta (DL). NH codeshares with United (UA) and THAI (TG). And JL codeshares with Alaska Airlines (AS) and American (AA). So, check itineraries on the NH, JL, DL, UA, TG AS & AA websites for tickets, looking specifically for flying via SEA.