Toss all the ingredients into a blender—banana, pineapple, milk, yogurt, and vanilla if you're using it.
Blend until smooth and creamy. Scrape down the sides if anything gets stuck.
Taste and adjust. Need it colder? Add some ice. Want it thinner? Splash in more milk.
Pour into glasses and enjoy right away. Or stick a mini umbrella in it and pretend you’re on vacation.