Preheat oven to 400 degrees. In a small saucepan, combine 3/4 cup water and sugar and cook, stirring, until sugar dissolves.
Cut each pineapple half crosswise into 3 pieces. Rub pineapple with vanilla seeds and place in a roasting pan in a single layer. Pour sugar syrup over pineapple to coat completely. Add vanilla pod to roasting pan and cover with parchment-lined foil.
Roast, turning pineapple every 30 minutes and basting with sugar syrup, until pineapple is caramelized and tender when pierced with the tip of a paring knife, about 2 hours.
Remove pineapple from oven, and let cool. Discard vanilla bean. Coarsely chop pineapple. Reserve 1/4 cup for topping Frozen Coconut and Roasted Pineapple Cake; transfer remaining pineapple and any accumulated juices to the bowl of a food processor and puree. Refrigerate until ready to use, up to 2 days.