There are many options which you could take but you just have to fingd out which one is the cheapest for you. Air Canada: Monteal-Vancouver-Tokyo. United: Motreal-Chicago-Tokyo. American-Montreal-Dallas-Tokyo. or if you prefer Asian carriers, Japan Airlines and ANA have flights from Vancouver. From Los Angeles, Japan Airlines, ANA, THAI Int'l., Singapore Airlines, and Korean Air have direct flights to Tokyo but you just have to make a connection flight to LA from Montreal.
You can check Air Canada, They also have flights to Tokyo from Montreal to Tokyo via Toronto.