Why?
A chemical change happens when the vinegar and baking soda mix together. They produce carbon dioxide. The balloon inflates as it becomes filled with this carbon dioxide.

So you started with a solid (baking soda) and a liquid (vinegar), but ended up with a gas (carbon dioxide).

What happens when you put the baking soda into the balloon, attach the balloon over the mouth of the bottle, and then shake the baking soda from into the balloon to inside the bottle? Wny?