A vivacious person is lively and spirited: a vivacious dancer might do a back-flip off the wall and then jump into the arms of her partner.
Some people are just naturally fun to be around; they sparkle, they animate any group they're part of, they're full of life. That, in fact, is the root meaning of the word: it's from Latin vivere "to live." It has more of a sense of playfulness than lively or animated. Even the most sedentary slob can feel revitalized in the presence of a vivacious soul.