Cut away the peel and pith of the grapefruit. Halve lengthwise, slice into half-moons, and transfer to a bowl with any juices, squeezing juice from the peel. (You'll need about 2 tablespoons of juice.)
Split the vanilla bean in half lengthwise using a sharp paring knife. Run the dull side of the knife blade down the length of each half, scraping up the seeds. Combine the seeds with the sugar and grapefruit juice. Toss until the sugar is dissolved.
Drizzle the syrup over the grapefruit and toss with the bananas and apple. Serve over the yogurt, topped with the chia seeds, if desired.