In a medium saucepan, combine the sugar with the water and lemon juice and bring to a boil over moderately high heat. Using a wet pastry brush, wipe down the side of the saucepan. Simmer the syrup undisturbed until it begins to brown around the edges, 4 to 5 minutes. Swirl the pan, then simmer until the caramel turns a medium-dark amber, about 3 minutes longer. Remove from the heat. Carefully add 1/2 cup of the cream in a steady stream. Stir with a long-handled wooden spoon to combine, then add the remaining 1 1/2 cups of cream. Stir in the vanilla. Transfer the caramel to a bowl and refrigerate until very cold, at least 3 hours or overnight.