This is the best way to reap the aromatic and taste rewards of quinces, best eaten when cooked.
Place the sugar and 12 cups water in a large stockpot, and bring to a boil. Scrape in the vanilla seeds, and add the pod. Squeeze the juice of the lemon into the sugar mixture, then add lemon halves.
Peel the quinces, and cut in half lengthwise. Add fruit and peel to mixture. Reduce heat to low, and simmer. Cook, turning often to keep quinces coated with syrup, until they are tender and pinkish, 25 to 35 minutes.
Remove pan from heat, and let cool. Remove quince halves, and cut each piece in half lengthwise. Using a melon baller or paring knife, remove cores. Pour liquid through a fine sieve, and discard solids. Store fruit with poaching liquid in an airtight container until ready to use.
Save any damaged fruit pieces for the sorbet.