Heat the sugar and water in a saucepan gently until the sugar dissolves. Halve the vanilla pod and scrape the seeds into the pan. Add the rest of the pod and the lime zest. Simmer gently for 10–12 minutes until the liquid is reduced and syrupy. Add the rum, if liked. Soak wooden skewers in cold water for at least 30 minutes before cooking the pineapple.