Zest the lemon.
If using fresh vanilla bean, split the vanilla bean.
In a large pot, place the quince, vanilla bean and lemon zest.
Add about 1 1/2 cups of water - just enough to cover the quince.
Simmer on medium low heat for about 20 minutes, until the quince are very soft.
Most of the water will have evaporated.
Drain and puree the soft quince with 1/2 cup of water in a food processor.
Measure the quantity of quince puree.
Place the puree and equal amounts of sugar back in the pot - about 2 cups.
Cook on medium heat until the sugar has dissolved. Add juice of the lemon and cook on VERY LOW HEAT, stirring often for 45 minutes, until thickened and rosy color.
Preheat oven to 200 F..
Line a baking dish with parchment paper.
Transfer the thickened quince to the oven and bake for 2 1/2 hours until most of the moisture is gone and it is quite firm.
Chill for at least 2 hours, but preferably overnight.