Preheat the oven to 325°F with a rack in the center.

Place the sugar, 3 cups of water, the lemon zest, and split vanilla bean in a large ovenproof casserole or deep skillet large casserole or deep skillet large enough to hold the pears comfortably lying down (or use 2 pans, dividing the ingredients). Set the pan(s) over low heat and simmer the liquid, stirring occasionally, until the sugar has dissolved and the flavorings have begun to infuse the syrup, 5 to 8 minutes.

Meanwhile, core the pears through their bottoms with the smaller end of a melon baler, then peel them with a vegetable peeler, moistening each with squeezes of lemon juice from the lemon halves to prevent darkening (do not rub their surface, however, because this will damage the fruit). Finally, slice a thin section from the base of each pear so it can stand upright.

Stir 3 tablespoons of the strained lemon juice into the syrup, add the pears, and roll them gently in the syrup with a rubber spatula (they won't be covered by the syrup).

Cover the pan(s) and place in the oven. Poach the pears until they're just tender but firm, about 15 minutes, turning them gently with a rubber spatula once during this period. To check for doneness, insert a wooden skewer into the cored middle and deeply pierce the upper, solid section of the pear (without going all the way through, if you can avoid it). If the skewer meets with more than a little resistance, continue cooking a few minutes longer.

Transfer the lemon strips and pears, supporting them with a rubber spatula as you life them by their stem, if there is one, to a platter. Loosely cover with plastic wrap and let the pears stand until cool, then refrigerate them.

With a teaspoon, scrape out the tiny black seeds from the center of the vanilla beans into the syrup; save or discard the pods.

Transfer the syrup to a small saucepan (there should be about 3 cups). Bring the syrup to a boil over medium-high heat and reduce it until lightly thickened and about 1 cup in volume, stirring occasionally, about 15 minutes, scraping down the sides of the pan from time to time to loosen any vanilla seeds. Stir in the remaining 1 and 1/2 tablespoons lemon juice, or to taste.

Let the syrup cool and refrigerate it until lightly chilled. Stand the chilled pears in shallow soup plates or bowls, stir the syrup to reincorporate the vanilla seeds, spoon it over the pears, slip 1 lemon strip into each bowl, and serve.

Note:
The pears retain the nicest shape if allowed to cool standing up.

Do-ahead Options:

• The pears can be Poached up to 1 day ahead and refrigerated in their unfinished syrup (finishing it the next day early enough for it to become lightly chilled).
• Remove the pears and syrup from the refrigerator 20 to 30 minutes ahead, so they are just lightly chilled when served.