VANNES   ONCE the capital of the independent Kingdom of Brittany, Vannes today is the préfecture of the département of Morbihan. The old quarter around the cathedral has been carefully preserved and pedestrianised and it is possible to walk on some of the ramparts above meticulously-weeded flowerbeds in what used to be a moat.   Between the ancient Postern Gate and the Prison Gate you look down on the unusually-shaped grey-tiled roofs of the Wash Houses curving along the river bank. This is the most photographed corner of Vannes. The nearby Cathedral of St-Pierre dates from the 15th century. Its high-vaulted knave is bordered with chapels. The Treasure on view in the chapter house includes chalices, pyxes, volumes of illuminated manuscripts and a 12th century wedding chest in vellum-covered decorated wood.     The atmospheric Place des Lices in the old town is crammed with market stalls on a Wednesday and a Saturday, and packed with people from the surrounding countryside as well as tourists. However there are plenty of large car-parks nearby.     The name Place des Lices indicates that this is the square where, in medieval times, knights fought tournaments 'in the lists'. François I was present at the great celebrations held here in 1532 to mark Brittany's union with France.     Some of the finest gabled and half-timbered houses may be found in Place Henri IV. The most intriguing of these is perhaps the House of Vannes in Rue Rogue. The façade is decorated with two jolly wood carvings of peasants referred to as Vannes and his Wife.     The Archeological Museum in the nearby château has fine prehistoric artefacts from all over the region. One advantage is that most of the items on display are also labelled in English. It is probably the best museum of its kind in Brittany.     Vannes is at the heart of the 'Little Sea' (which is the meaning of Morbihan in Breton) and from its harbour pleasure boats sail the sheltered waters of the Gulf of Morbihan, with its scatter of islands. There are sheltered beaches from which to bathe and enjoy water-sports.