In a medium saucepan, combine half of the raspberries with the vanilla bean, sugar, raspberry liqueur and water; simmer for 4 minutes, then remove the vanilla bean. Transfer the berry sauce to a blender and puree. Strain the sauce into a clean saucepan and return to a simmer. Put the remaining raspberries in a heatproof glass bowl and pour the hot sauce on top. Stir in the lemon juice and refrigerate for up to 2 days. Rewarm before serving.