Vin doux naturel translates as 'naturally sweet', but this is hardly the case, we are talking about wines made using 'mutage', the addition of grape spirit to the grape must, added before completion of fermentation. This action kills the yeast, and the unfermented sugar causes the wine to be sweet.
With 'mutage', however, the yeast action is halted by the addition of alcohol, resulting in what is really a blend of wine, unfermented grape juice and added grape spirit.
The most common wine styles involve Muscat usually 'Muscat a Petits Grains', as in Muscat de Beaumes de Venise and Rasteau AOC, and also Grenache, as in Banyuls an appellation of Roussillon.

 

This region might be regarded as the birthplace of the vin doux naturel method in France, as it was here, in the 13th Century, that Arnaud de Villeneuve, of the medical school at Montpellier University, perfected the technique. Arnaud a renowned doctor demonstrated that the addition of spirit to the must halted fermentation, and so won a patent from the king of Majorca, who then ruled Roussillon.
Despite this great success, this particular use of grape spirit was of no great interest to Arnaud; like many physicians, he valued the relatively new process of distillation because of the medicinal properties of the alcohol obtained. Yet centuries later, Arnaud's method of mutage remains essentially unchanged.
With vin doux naturel (VDN), the spirit accounts for up to 10% or the total volume, but it is traditionally a very strong spirit that is added, often around (190 proof); as a result the total alcohol content of the finished product is typically 15%. Nowadays, the grape spirit is likely to come from the same source, the European wine lake, rather than local distillation, but whatever the source, the blending is such to give the final desired ABV in keeping with local tradition and, of course, wine laws. The resulting wines are usually 15% to 18% alcohol but can range as high as 21%. VDNs vary in sweetness, with white wines generally being sweeter and less alcoholic than reds.