We went on the Vision to Mexico last year. Nice ship, although there is one area of the buffet that is inaccessible to wheelchairs, and the seating for wheelchairs in the show room sucks (all the way in the back, poor sight lines, can' t sit with the rest of your party, etc.). The cabin we had was very nice for 3 people with a great accessible design and lots of storage space (we had a balcony) and was right by the elevator.
Definately take both your power and manual chair. A power chair can break down, and if you do have to tender (sometimes unanticipated) you will need the manual chair.
We went to Alaska with Princess, which usually is totally worthless for helping you find accessible tours, but they managed in all our ports. What other ports do you have beside Juneau? We had a very nice tour in Juneau that we arranged through the cruise line (1/2 day) which included the Mendenhall glacier, the Alaska State Museum, a native plant botanical garden (not accessible) and the salmon hatchery. You are right that the town (at least the shops, etc.) are right at the pier so an easy walk. Mostly we found curb cuts and ramps into shops. It is the cable car (not a trolley) that has problem wtih seeing out. It goes up to the top of a mountain near the pier, and yet, the walls are too high to see out from a seated positon. You can get out at the top and see the view there though.
We had no problems getting an accessible cab in Vancouver...I would guess that Victoria would be similar.