Place quince and lemon in a food processor; pulse 10 times or until finely chopped. Place quince mixture, sugar, water, and vanilla in a large, heavy saucepan. Bring to a boil; reduce heat, and simmer 55 minutes or until reduced to about 3 1/2 cups. Cool; pour into an airtight container.
Note: Refrigerate marmalade in an airtight container up to two months.