What a fabulous trip you are planning. I did a six week trip around Europe with two people that I didn't know and that went much better than a ten week trip with one of my best friends.
My best advice to you is to realise ahead of time that the going will get tough some times. One of the best ways to break things up and have a little breathing space is to take some alone time - and cities offer the perfect opportunity. As long as you feel comfortable split up for the day - it also makes for some interesting conversations over meals.
As for your destinations? As a Vancouverite (I am guessing in BC, Canada - you can ignore this if you mean in Oregon) I can wholeheartedly recommend a visit. The best way to break up the trip btw. there & Chicago would be to drive through the southern part of BC (Hwy #3) - its is gorgeous (be forewarned though that this is a meandering 2 lane highway, you won't be able to go much faster than 65 mph for most of the trip), then in Southern Alberta cut south through Waterton National Park (AB) into Glacier National Park (MT), which puts you in a good position to access Yellowstone (and the NW entry to the park is supposed to be spectacular).