by 
BlackHat
ANSWERS: 2
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Maybe there's an endothermic reaction going on being sped up by the shaking, while the heat conduction is held about constant (apart from you holding the bottle). Since acetic acid is weak, I'd imagine that the endothermic reaction in process is the dislodging of the Hydrogen atom. However, I haven't heard of vinegar cooling off as it is shaken, so I could be mistaken.
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I think it works with plain water, too. Nothing special about vinegar. The liquid is a better conductor of heat than air is. So where your hand touches the bottle, if there's air on the other side it feels warm & if there's liquid on the other side it feels cold. Slosh it around and the liquid absorbs heat from everywhere your hand touches the bottle. After a while, of course, the liquid heats up & no longer feels cool.
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