I think you're onto something here.
While I have not done exactly what you describe (my starting point was different), last year I flew out of Vancouver to London and the Air Canada airfare was significantly lower, to London, than other airfares that I could find.
However, I would say--based on airfare research I've been doing recently for another trip (which is not a Seattle departure and not to London)--that whether flying out of Vancouver BC will always be less expensive, for every traveler's itinerary, depends on:
1) your starting point (e.g., Portland versus Seattle)
2) your destination in Europe (e.g., Vancouver to London versus some other Europe destination)
3) what other airlines' are offering for the traveler's specific itinerary to Europe.
So I would recommend, as always, checking all the usual airfare sources, before deciding.
It sounds like you've done that and, for your trip, flying out of Vancouver makes sense.