You need to be very careful with this one, and (depending on the purpose of your document) you may well find the best solution is not to translate it at all, but to add an explanation.

A 'vin doux naturel' is a naturally sweet wine, usually made from grapes that have suffered from the 'noble rot', which has made them over-ripe and super-sweet

A 'vin liquoreux' has had some sugar added to it to make it sweeter (though I believe this is a naturally-occuring grape-related sugar, and not just any old cane sugar) --- this is NOT the same as a 'fortified wine'

A 'vin cuit' is what we usually translate as 'fortified wine' (e.g. port or sherry), and it is a wine that has been cocnentrated by partial evaporation of the water content, with or without the addition of extra sugar.

I'm not a wine expert myself, so some of my explanations may not be technically quite spot on, but I have this information on good authority from people who DO know what they're talking about, so please be warned --- at any rate, it is important to be aware that the 3 types mentioned above are NOT AT ALL SYNONYMOUS --- Sauternes is not the same as Banyuls / muscat / sherry / port / pineau etc. etc.

&

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Note added at 28 mins (2005-10-04 15:39:53 GMT)
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It's important to note that this is not just some arbitrary descriptive term, like 'sweet wine' or 'sparkling wine' or 'dessert wine', but an actual technical term with a very specific meaning in the industry -- and woe betide you if you mis-attribute it to the wrong type of wine!


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Note added at 1 hr 21 mins (2005-10-04 16:33:06 GMT)
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A little bit more information (but I'm starting to get myself confused now!)

Sugar is NEVER added to make the wine sweet.

Either, you start off with super-sweet grapes, and fermentation stops when the yeast can no longer ferment any more sugar; that way, you get alcohol + residual sugar (= Sauternes, Barsac, Jura...)

Or, you let fermentation take its natural course, then sweeten it at the end by adding unfermented grape juice

Or, you deliberately stop the fermentation before all the sugar has been used up.

These different techniques produce the different varieties of French (and other) sweet wines

They are quite different from the techniques used to produce 'vin cuit', wines that have been fortified in various different ways...



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Note added at 1 hr 41 mins (2005-10-04 16:52:34 GMT)
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Here are various trawlings from glossaries on Google:

LIQUOREUX
Vins blancs riches en sucre, obtenus à partir de raisins sur lesquels s'est développée la pourriture noble, et se distinguant entre autres par un bouquet spécifique. [Dusty adds: i.e. Sauternes etc.]

V.D.L.
Vin De Liqueur; vin doux ne répondant pas aux normes réglementaires des VDN, OU vin obtenu par mélange de vin et d'alcool (Pineau des Charentes).
V.D.N.
Vin Doux Naturel; vin issu de muscat, grenache, macabeo et malvoisie, et correspondant à des conditions strictes de production, de richesse et d'élaboration.
VIN DOUX NATUREL (VDN) : vin pour lequel on procède au mutage (arrêt de la fermentation alcoolique, par l'adjonction d'un certain pourcentage d'alcool de rétrocession compris entre 5 et 10 % du volume des moûts). Les plus célèbres sont Banyuls, Maury, Rivesaltes, et les Muscats de Rivesaltes, frontignan, Saint-Jean-de-Minervois, Lunel et Beaumes-de-Venise.
V.D.N.

Vin Doux Naturel.

Vin cuit
Vin obtenu à partir d'un moût qui a été concentré par chauffage.

Liquoreux
Se dit d'un vin blanc onctueux, moelleux et présentant beaucoup de sucres résiduels.

Vin doux naturel : un vin muté à l'alcool pendant fermentation.


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Note added at 20 hrs 47 mins (2005-10-05 11:59:15 GMT)
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In answer to Jane's plea:

if this is a technical context, I would be inclined to leave it untranslated as a French term, and just add an explanation.

If, however, this is an informal context, I think 'sweet (white) wine' would probably be sufficient, though could be amplified, depending on the exact context.