For a nonalcoholic version, omit the vodka, and top each drink with seltzer. Garnish with Pineapple Flowers.
Preheat oven to 325 degrees. Put 1 cup pineapple juice and the sugar into a roasting pan set over two stove burners on medium heat. Scrape vanilla seeds from bean into pan with the tip of a paring knife; add bean to pan. Whisk until sugar has dissolved. Add pineapples, and cook until they begin to release juices, about 10 minutes. Place pan in oven; cook, basting occasionally with pan juices, until edges of slices are deep golden brown, 50 to 60 minutes. Discard vanilla bean. Let cool completely.
Puree pineapples and pan juices in a blender, then pour through a fine sieve into a heatproof airtight container. Refrigerate until cold (up to 3 days).
Sprinkle caramel dust inside twelve 8-ounce glasses. For each cocktail, combine 4 ounces roasted-pineapple puree, 2 ounces pineapple juice, and 1 ounce vodka in a cocktail shaker; add ice. Shake 30 seconds, then strain into a prepared glass.