Heat the water, sugar, maple syrup, lemon and vanilla bean (being careful to scrape the seeds first and add them to the liquid) in a large heavy-bottomed pot on medium–high.
Meanwhile, working on the quince one at a time, peel, cut in quarters, then cut each quarter in half again. Then trim and remove the fibrous core/seeds from each one and pop the slices into the warming poaching liquid ASAP. Don’t worry about browning. It actually disappears during the cooking.
When all the quince are in the poaching liquid, bring them to a boil, then reduce heat to simmer.
Cut out a large piece of parchment and poke a walnut-sized steam hole through the middle. Fit it over the pot, using the lid to fit it in place. Simmer the mixture covered with just this piece of parchment for about 45 minutes to an hour, until the quince slices are fork tender.
Let them cool slightly before serving over vanilla ice cream with a drizzling of the poaching liquid.