Can I rent a large MPV like Toyota Sienna from Vancouver and Montreal? We are a group of six people and there are only Chevrolet Tahoe on the website ?

You can first book a similar car model, and then communicate with the staff when you arrive at the store, telling them your ideas and requirements, and asking them to help you coordinate. Because often times, the car you get in your hands may not be the one you pre-ordered, and they are usually cars of the same level.