When we have out of town guests that want to do the Vineyard, we normally do the Island Queen out of Falmouth to Oak Bluffs. Plenty to do in OB - tour the tabernacle camp ginger bread houses, ride the oldest merry-go-round - you really don't have to go anywhere else. But Edgartown is close enough to do both in a day and catch a ferry back.
For the Rock, I'd definitely spend the extra few bucks on the fast ferry. 1 hour dock to dock. The Whaling Museum is excellent and there's great food everywhere. The Rose and Crown pub downtown has excellent chowder.