I recommend using an actual vanilla pod for the best (and more natural) flavored vanilla syrup. My plan B would then be to use a vanilla bean paste. And my last option is vanilla extract (added after taking the syrup off the heat). However, technically, any would work. Use one tablespoon of vanilla bean paste per vanilla pod (at the beginning) + 1 teaspoon of vanilla extract (at the end). For vanilla extract, add it at the end, after removing the syrup from the heat. If you plan to use vanilla extract, use a high-quality option with real vanilla. Low-cost versions often taste very artificial and like chemicals. It's best to use a homemade version or a brand like Nelsen-Massey.