I will be visiting in May and will be taking this ferry to Olympic National Park in my rental car. My wife will be with me. Do I purchase vehicle and driver ticket, AND a passenger ticket for my wife?

If you are leaving from Seattle area you will need to take Seattle to Bainbridge or the Edmonds to Kingston, not this ferry. If you are planning to go the ONF from Whidbey Island you will Coupeville- Port Townsend and ferry and you will want to make a reservation for that. You can do that they the Washington State DOT website. The site will walk you through it. The Coupeville- Port Townsend ferry is the o ly that requires a reservation. Otherwise the attendant at the ticket booth will charge you accordingly as some runs don't charge for passengers ( its a kind of a complicated ticketing formula they use). You can just pay when you are at the ticket booth for all of the runs as it is first come first serve & no advance purchase except for the Coupeville ferry. The WSDOT website should have all the information you need and a phone number to call. Enjoy your visit and expect waits times that might be long during peak times.

I suggest a round trip ticket and will be car/driver plus passenger.