Combine the lime juice and zest, sugar, water, and vanilla extract in a small saucepan. Stir to combine and over high heat bring to a rolling boil. Once the mixture has boiled reduce the heat to low and allow the mixture to simmer for about 10-15 minutes until the sugar is completely dissolved and the mixture has a slight syrup consistence. Remove from heat and allow to steep for about an hour. Strain the zest and store in a glass container with a tight seal until ready to use. Makes about 2 cups of lime simple syrup