Put the softened butter and 1⁄4 cup of the sugar in a medium mixing bowl. Using the back of a knife, scrape seeds from 1 of the vanilla beans into the butter, then mix well to make a soft, creamy vanilla butter. Reserve vanilla pod. Set vanilla butter aside.
Combine remaining 4 cups of the sugar and 4 cups water in a wide medium saucepan; cover and boil over medium-high heat, swirling pan once or twice, until sugar dissolves, 4-5 minutes. Meanwhile, avoiding the white pith, cut the rind off the lemon in long, wide strips using a vegetable peeler or a sharp paring knife and set aside. Cut lemon in half. Peel apples, then cut out the core, leaving each apple intact. Squeeze juice from lemon halves over apples to keep them from discoloring. When sugar has dissolved, reduce heat to maintain a gentle simmer, then add the remaining 2 vanilla beans, the reserved vanilla pod, the reserved lemon rind, and the apples. Poach, partially covered, until apples are just tender when pierced with a skewer, about 15 minutes. Transfer apples with a slotted spoon to a plate and set aside.
Pour 1 1⁄2 cups of the poaching liquid into a small saucepan and simmer on medium-high heat until thickened and syrupy, 10-15 minutes. Set vanilla syrup aside to cool. Reserve remaining poaching liquid.
Meanwhile, preheat oven to 350°. Cut crusts off bread slices and chop crusts into a medium-fine crumb. Spread crumbs out evenly on a baking sheet and bake until deep golden brown, about 10 minutes. Set aside to cool. Roll out each slice of bread with a rolling pin until there is no spring left in its crumb and it is completely flattened. Butter both sides of each slice with some of the reserved vanilla butter. Press 2 slices of bread, one on top of the other, into each of four individual 4'' tart molds, allowing the corners of the bread to extend over the edge of the mold. Set tart molds on a small baking sheet and bake until bread is brown on the edges and golden in the center, about 30 minutes.
Place poached apples upright in the center of each tart crust. Combine bread crumbs and remaining vanilla butter in a medium mixing bowl, then spoon into hollowed-out centers of poached apples. Spoon some of the reserved vanilla syrup over each apple and return to oven to bake until heated through and slightly browned on the edges, about 30 minutes. Serve apples warm, drizzled with some of the reserved poaching liquid.