Wine is not necessarily vegan. It is not even vegetarian. If you are scratching your head wondering what the difference is, well, vegetarian diets include some animal products such as milk or cheese, but vegan diets exclude all foods derived from animals. Wines are made from grapes so they must be vegetarian and vegan. But wines rely on a process known as fining and for that a few ingredients are used which make wines non-vegan and perhaps non-vegetarian too.
Fining is a process through which wine is clarified. During the winemaking process, the fermented grapes and the resulting liquid tend to have tiny molecules and it is hazy. The wines we drink are clear. The tiny molecules are proteins, tannins, phenolics and tartrates. These are naturally occurring substances and completely harmless. Wines can get fined naturally and become stable, but it takes a long time. Winemakers use fining agents to speed up the process. These fining agents are usually casein, albumin, isinglass and gelatin. Casein is a protein found in milk. Albumin is derived from egg whites. Gelatin is an animal protein. Isinglass is a protein derived from fish bladder. It is perhaps clear as day that most wines are not vegan if they have been subjected to a fining process involving these agents.
Vegetarians may not have an issue with casein. Many vegetarians do not find albumin or egg whites either. However, vegans avoid both and hence a wine needs to be refined with other agents than these traditional options. Many winemakers have started using bentonite and other clay-based agents. Some winemakers are using activated charcoal to refine their wines. These materials as fining agents make a vegan wine. Whenever you choose a wine, find out what the makers have used as the fining agents and you will know if it is vegan.