Yes, they will set up an umbrella for you as long as it is not too windy.
Yes, the cabana beds are extra. Instead of reserving a Vibe pass, you would reserve a cabana which comes with two vibe passes. This cabana would then be yours all week (or however long the cruise is) and no one else will be using it. Actually, some will try to use it, but the Vibe staff will immediately let them know they cannot use it.