- Place tangerines and lemon in a large, heavy-bottomed saucepan and add cold water to cover. Bring to a boil, then reduce heat and cook over medium-low heat for 45 minutes, or until the peels are tender when pierced with a fork.
- Lift out the fruit, reserving the cooking water. Quarter the fruit, reserving the seeds.
- Pull away the peels from the cooked fruit and slice finely. Using your fingers, pull out the membranes that separate each section of fruit.
- Place seeds and membranes on a 12-inch square of double-thickness cheesecloth. Gather up the corners and tie them shut with kitchen string.
- Place the cheesecloth bag, the peels, and the fruit pulp in a large mixing bowl. Scrape in the seeds from the vanilla bean and add the bean pod. Add the reserved cooking water, cover, and let stand overnight.
- Place mixture in a medium, heavy-bottomed saucepan. Bring to a boil.
- Add the sugar gradually, stirring with a wooden spoon, until dissolved. Reduce the heat to medium and simmer for 15 to 20 minutes, stirring frequently, until a candy thermometer reads 220ºF. (If you have not soaked the fruit overnight, you may need to cook the mixture for about 1 hour, to further soften the fruit.) If the marmalade “wrinkles,” it is set.
- Remove and discard cheesecloth bag. Skim off any scum. Let stand in the saucepan for 5 minutes before ladling into hot, sterilized canning jars, leaving 1/4 inch of headspace. Seal.
- Process the jars in a hot water bath for 5 minutes. When thoroughly cool, label the jars. Store in a cool, dark place.