From the Original Bakery, you’re best bet is to walk down to the ferry dock and walk on the ferry. If there is a bus on the ferry headed onto Vashon, you can board the bus on the ferry and ride the bus as it departs the ferry onto Vashon. Otherwise, you can walk off the ferry and catch the buses which will be on the dock just past the terminal.
Just be sure to check the 118 schedule to make sure there’s a bus when you need it. When I used to be a Vashon resident and regular ferry commuter, the buses tended to be concentrated during commuter hours and less frequent during other hours throughout the day and evening.