Actually headspace does create vinegar.
As you might remember from high school (though people tell me that education could be better in the USA) oxygen can oxidize ethanol (alcohol) to acetaldehyde and then acetic acid which is vinegar.
1 gallon of headspace will oxidize about 2 mL alcohol to vinegar which isn't much. I doubt you'd notice the taste frankly. I believe that acetobacter needs oxygen to make vinegar. Anyone know about that?
"It has served us well, this myth of [guess who]."
- Pope Leo X