In a medium saucepan, combine the sugar, water, and white vinegar. Place the saucepan over medium heat and stir constantly until the sugar completely dissolves. Bring the mixture to a gentle boil. Reduce the heat to low, then add the tied bunch of fresh mint to the syrup. Allow the syrup to simmer for 20-30 minutes, or until it has slightly thickened to a syrupy consistency. Do not let it become too thick, as it will thicken further upon cooling. Remove the saucepan from the heat and carefully take out the mint bunch. If using, stir in the rosewater at this stage. Let the Sekanjabin syrup cool completely to room temperature. Once cooled, pour the syrup into a clean, airtight bottle or jar. Store in the refrigerator for up to several weeks. To serve, dilute 1 part Sekanjabin syrup with 3-4 parts cold water, or to your desired taste. Serve over ice, garnished with fresh mint leaves or even thin cucumber slices. Traditionally enjoyed with fresh lettuce leaves for dipping.