Why did my vinegar not create the traditional volcano?
Acetic acid can be decomposed by methane-producing bacteria. Note that the citation is for soil bacteria, but these methanogens can be found almost anywhere.  Indeed, there is an entire microbial community in vinegar, including nematodes (worms). The more &quot;natural&quot; the product, the more organic material to nourish the microbes and to speed decomposition.
That said, check the labels for acetic acid concentration when purchased. The nominal concentration for distilled vinegar is between 5% and 10%, but some types of vinegar may be only 4% acetic acid.