"Must see" is very much in the eye of the beholder. However, someone recently asked that question here on the forums, and you might find it interesting:

tripadvisor.com/ShowTopic-g154911-i221-k2035…

As for the rest of your questions, I think you will find the TripAdvisor website to be helpful. Start by clicking the OVERVIEW and BEFORE YOU GO links (left hand menu bar) on the pages for Banff, Banff National Park, and Vancouver. That will lead you to interesting things like the top-rated hotels, restaurants and attractions for each area, as well as the many TripAdvisor Traveler Articles written about each area by TripAdvisor members like you. Scroll down to the bottom of some pages to find links to related articles on other websites, such as Fodor's and Frommer's. Use the forum search box to find the many old postings regarding things like the Rocky Mountaineer and Via Rail trains. And once you have more specific questions, feel free to post them on the forums.

You can save money by flying from Seattle to Sandiego.Alaska air and Jet Blue come to mind.You also can take the Amtrak from Vancouver to Seattle,a nice 4-6 hour trip.try the Via Rail website for train trips in Canada too.You could try the VIA RAIL train from Jasper to Vancouver and see if you can still get a sleeper.

Hello from Australia.Make sure you walk to the end of Lake Louise and go to the top of the rockpile at Moraine Lake for some great photos.Johnston Canyon,Emerald Lake,Peyto Lake,Sulphur Mountain,Vermillion Lake at sunset,Bow Lake,Bow Falls,lake Louise gondola ride,Takkakaw Falls were other places we found to be really good around Banff.If you can get up before sunrise and go to the two lakes you mentioned for some great photos.At Jasper Maligne Lake(spirit island) and Cadell Pond at Mt Edith Cavell are definately must sees.Jasper Tramway is also good but get there early for this one otherwise you may have a long wait.Jasper is a beautiful town.Besides the above you have got the icefields parkway which is absolutely awesome to drive along,it has some amazing scenery.Have a great trip,it is a fantastic place to visit.