I’ve done similar to Robespierre for August. Managed to get reward seats in CW for LHR > SEA and return. Although these were booked around 340 days out and with an Amex Premium Plus Companion Voucher (Vancouver availability was sparse, even by doing this).

Rather than fly, we’ve decided to take the Amtrak to Vancouver as it’s meant to be a very scenic route. We’ve also factored in returning from Vancouver to Seattle 3 nights before the return flight and taking the opportunity to see the city. IMO rather than making flying into and back out of SEA a necessary evil we’ve just made it part of the holiday.