Do not apply vinegar in this fashion; outside of specific treatments used to treat weeds organically, vinegar (or more appropriately, acetic acid, since it's often much stronger than what we consume in food) should not be applied to the soil. It would also probably break down into fairly dilute or inert ingredients after a period of exposure to sunlight, soil chemistry, or soil microbes, such that it would no longer function like a fungicide (assuming it would have any impact at all). Worst-case, a heavier dose might, at least temporarily, lower soil pH to a level not tolerated by the roots of crops you'd be growing.

A wide variety of fungi exist in the natural environment that don't cause plants serious infections when the plants are not particularly vulnerable due to stress or the right weather conditions. The factors making them vulnerable will depend on the plant and the environmental conditions, but often include being planted too densely (or any situation where air circulation around their foliage is reduced, which keeps leaves from drying off as quickly), not being grown in enough sunlight (for sun-loving plants), and being over-watered or having their leaves wet-down often. Wet leaf surfaces are generally easier for fungal and bacterial spores to infect. Some fungi, like the very common and comparatively prominent powdery mildew, only require high humidity to thrive instead.

Sharing photos of the plant's symptoms next time this ailment appears would help us diagnose it for you so you know what particular disease is present and how to treat for it. Sometimes fungicide use is beneficial if a problem is caught early, though in other cases other measures not involving chemicals is more effective or safer for pollinators.

Miri