Honestly a rental is your best bet since you can travel on and off the ferry. Finding rides on Vashon won't be as easy as the city, but you can manage to find an Uber or Lyft, just might take a while.
Not sure about Vashon, but you can take a taxi or uber to the Fauntleroy Ferry Terminal
King County Metro Transit trip planner, and Washington State Ferries websites online will be able to help figure out transit to the island. A rental car best to island from Airport and on island. Transit on island is sparse Two ferry runs to island. Fauntleroy and Pt Defiance. Fauntleroy is largest and closest to airport. Cars line up on Fauntleroy Way SW curb along Lincoln Park. You must go to the north end of the line with your car, no left turns into the terminal dock. This run is the triangle, 3 stops Fauntleroy Vashon Southworth and uses 3 boats on peak hours. Late afternoon evening lines at Fauntleroy are long. Right now Nov 2019 is 2 boat schedule due to maintenance schedule of the fleet. Lines are epic. As most public infrastructure these days, our fleet is funded less than optimaly. Bus stop at dock entrance, also uber. You will probably need a car on the island. Crews do best can with what have and a daily ride across the water is the opposite of a jammed freeway crawl
Is taking a rental car on the ferry difficult? Does it take awhile or guaranteed pretty much if you show up? I picture two cars and a little boat....
Thanks! Your answer is awaiting moderation.