If we drive our car from Vancouver to Seattle, and fly from Seattle to LA , we need leave our car around 7 days, how much will cost and do we need purchase the ticket early?

You definitely want to call and contact the airport or wherever you choose to leave your car you need to contact that business because hours can change on the weekdays and weekends

There a long term parking available .the cost is variable .

There are so many parking outfits, cannot give an estimate..