Vinegar by itself can be opened and stored in the pantry for more than a year before anything noticeable happens. White vinegar will last indefinitely. When vinegar is mixed with other ingredients, however, this protection is diminished or may disappear completely. This applies to vinegar-based salad dressings, even if they're stored in the refrigerator.
Vinegar as a Preservative
Vinegar is often used as a food preservative because of its low pH. Low pH equals high acidity, and harmful bacteria typically cannot grow in an acid environment. Citric acid or ascorbic acid can also be used in place of or in addition to vinegar as a food preservative. Though most vinegar can last for five to 10 years, the flavor and consistency of vinegars other than distilled white vinegar will become less appealing after about a year.
The Role of Other Ingredients
The reality is that vinegar-based salad dressings are not just vinegar. And, while vinegar can extend the shelf life of other foods, they still won't last nearly as long as the vinegar on its own. Even with vinegar as an ingredient, the addition of dairy in ranch dressing or eggs in mayonnaise will cause the dressing or condiment to spoil more quickly. Another consideration is the addition of fresh herbs such as dill or garlic because these ingredients also start breaking down once they're exposed to air. Without these extras, Italian dressing and other vinaigrettes will last somewhat longer.
Commercial Salad Dressing
Most commercially made salad dressings use the same types of preservative ingredients that homemade ones do. Vinegar, sugar, citric acid (usually lemon juice), ascorbic acid and salt are common to both kinds of salad dressing. The difference is that commercial salad dressings are usually pasteurized to comply with safety regulations and then stamped with a "best by" date, which varies from one to four months. If the smell of the dressing is questionable, that date comes in pretty handy.
Homemade Salad Dressing
Of course, there is no such "best by" date stamped on homemade vinegar-based salad dressings. They're typically not pasteurized either. If kept in the refrigerator, a good guideline for homemade salad dressing is two weeks for a simple vinaigrette and about one week for dressings that contain dairy or fresh herbs. Dressings containing raw garlic should be eaten within a few days because of the risk of botulism.