Found this on the Expedia Fare Calendar - IAD-BKK:

VNXAPTH is the fare basis for an IAD-BKK trip - 705.00 - 6 day minimum stay.

You can stopover in SEA -

STOPOVERS - PERMITTED-3 1 FREE NOT PERMITTED IN HONOLULU 1 FREE ADDL 2 AT 100.00 USD

If it co-incides with your seasons:

SEASONS - 28DEC-31MAY
DEPARTURE OUTBOUND TRANSPACIFIC SECTOR