Most bacteria in homes grow in biofilms. Like the whitish stuff that accumulates on your teeth during the day or slimy dirt on plugholes. A biofilm is a three-dimentional structure, usually made of polysaccharides, which protects the bacteria that are inside. It can even form quasi-differentiated structures, similar to fungi organs. Biofilms are extremely resistant (a nice picture), even if you boil, dip in chlorine or any other way sterilise a slimy plug, you will not kill all the bacteria. On the other hand, deep inside the biofilm bacteria have little oxygen and die or form spores, which, inside the biofilm, are even more difficult to neutralize. Additionally, even if the bacteria themselves are hamless, the chemicals released after their death might be bad for humans.
Bacterial biofilms always grow on the border between two substances - solid and liquid, solid and gas (air), liquid and gas. Maybe you have seen a film on top of an old broth? This is exactly a biofilm. Look closely, and you will see, that the broth is still clear, not murky, but on the bottom of the container is a layer of dead bacteria. Biofilms don't care about the acidity of the medium below, because the polysaccharide cover shields the bacteria from direct contact. Biofilms may form on any surface, that gives them some water and nutrients.
Not every kind of bacteria can form a biofilm, but certainly any bacteria that can form a biofilm can make it on your refrigerated veggies. Pathogenic bacteria that form biofilms include (but are not limited to): Pseudomonas aeruginosa, Staphylococcus aureus, Bacillus cereus and subtilis, E. Coli. Any liquid or solid under a biofilm should be considered piosonous.
A friend of mine who studies bacterial biofilms needs about a week to grow a thick biofilm on a petri dish or in a test tube, but eating that biofilm will be bad for one's health much sooner. On the other hand, we don't inoculate food on purpose, it just happens. Also, people's resistance to such bacteria may vary even between otherwise healthy humans. There is no single answer when your veggies pose a health hazard, so you need to go with tips from good receipes - leaving veggies submerged in vinegar is usually safe for a month, but if you see some film forming on the surface earlier than that, just throw it away.