Answer:
"When the two ingredients are mixed, hydrogen ions ( H+) from the vinegar react with the bicarbonate ions (HCO3- ) from the baking soda to form a new chemical called carbonic acid (H2CO3).
H+ + HCO3- ---> H2CO3
The carbonic acid thus formed then immediately decomposes into carbon dioxide gas (CO2)and water (H2O)."
(Yes, copy+paste is unoriginal...)
http://www.apple-cider-vinegar-benefits.com/baking-soda-and-vinegar.html