Aye, scrapers can be a bit distructive, not just in themselves but coupled with the fact that freezing temps can make the vinyl a little brittle.
As far as cleaning go, the graphic is a bit like a fresh coat of paint - it takes a couple of days, though more in the cold, for the adhesive to 'dry', after which it should be fine to wash over. It then takes a few months for the adhesive to fully 'cure' after which it would probably hold up to mild jetwashing (assuming there's been no nicks or lifting in the meantime).