Booking is open on the Cascades starting in January 2022. I am booked in March EVR-VAC to connect with the Canadian.
@Triley has said here that Amtrak/Washington state are in discussions with Canada on how to restart the service, especially given Canada's random testing requirements. Maybe she has an update for us?
None of the bus services are running. Amtrak's Thruway only runs to Bellingham, neither Greyhound, MegaBus, nor QuickCoach are running across the border as of now. There is no ferry service directly between Seattle and Vancouver, both being on the mainland and well connected by highway and rail. You can always take the Victoria Clipper, which is operating to Victoria, then BC Ferries to Vancouver. There is a bus that uses the ferry that can take you from downtown Victoria to downtown Vancouver, though I don't recall the name offhand. That is the long way around, though.