"Mother" of vinegar (cloudy vinegar) will naturally occur in vinegar products as the result of the vinegar bacteria itself. Today, most manufacturers pasteurize their product before bottling to prevent these bacteria from forming mother while sitting on the grocery store shelf.
After opening, you may notice mother beginning to form. Vinegar containing mother is not harmful or spoiled. Just remove the substance by filtering and continue to use the product.
The Vinegar Institute conducted studies on how long vinegar will last and it confirmed that its shelf life is almost indefinite. Because of its acidic nature, vinegar is self-preserving and does not need refrigeration.
White vinegar will remain virtually unchanged over an extended period of time and while some changes can be observed in other types of vinegars, such as color changes or the development of a haze or sediment, this is only an aesthetic change, unless the vinegar has become contaminated. The product should be safe to use unless contaminated.