All 3 Cities are worth a visit
Your limiting factor with II is a lack of resorts in the area. Victoria has only 2 timeshares. One is a Worldmark that may have II affiliation but didn't show up on their resort list. We just spent 2 nights in Victoria a couple of weeks ago for the 3rd time and had a great time. The city core is fairly small and there is a lot you can do. Butchart Gardens is a few miles away and worth a visit. If you are into museums, you would need more than 2 nights.
Vancouver has 4 resorts listed under II. Only the Worldmark has more than 1 bedroom. Again there is a lot to see. We have actually stayed in Birch Bay twice and made trips across the border to Vancouver. There are several timeshares there including a Worldmark. It's 15 minutes to the border, but you have to factor in the time to get across the border that can be up to an hour, then another 1/2 hour into Vancouver. Several areas to check out including Stanley Park, Gastown, Granville Island, Dr. Sun Yat-Sen Park and Classical Chinese Garden come to mind. If you should end up in Birch Bay, the town of Blaine WA is worth a day. If you stayed in Vancouver, the ferry to Vancouver Island will get you to Victoria.
Seattle is spread out and traffic challenged but again lots to see. Tripadvisor is my main guide before any trip. Maybe your Marriots will get you some nights there.