Wash the lemon, rub dry and cut into slices. Put wine, currant juice and liqueur in a large pot. Slice the vanilla pod lengthwise. Add lemon slices, sugar and vanilla pod to the juice-wine mixture. Bring to the boil while stirring occasionally.
Simmer on low heat until the sugar has dissolved. In the meantime, peel the pears without removing the stalk and sprinkle with lemon juice. Place in the pot and cook over low heat for about 30 minutes. Turn occasionally so that the pears are evenly red all around. Leave to cool in the syrup and steep for at least 2 hours