The only thing I have come across with a vinegar smell is some types of coolant. If you remove radiator cap a take a wiff of what is in the radiator and get the same smell, I would think there may be a leaky heater core.

It is somewhat common for the evaporator to develop a slow leak. The PAG oil in the A/C system has a unique odor but I wouldn't call it a vinegar smell, more of a musty odor. Hard to decribe.

Gary.