Combine the brown sugar, water, rum, and vanilla beans in a saucepan. Squeeze the lemon juice into the pan. Stir to combine the ingredients and bring to a simmer. Add the pineapple and stir until well mixed. Simmer until the liquid has reduced to a syrupy consistency and the pineapple is soft.
Let cool and serve as an accoutrement to yummy desserts like goat cheese cheesecake!