Bring beet juice to a simmer with the 1/2 vanilla bean in a small saucepot over medium heat. Reduce juice, skimming constantly, to a syrup. Scrape the seeds from the vanilla bean into the syrup and discard the pod. Place each plantain slice, cut side-up, on a cookie sheet and roast in a preheated 400-degree oven for 3 to 4 minutes, or until heated through. Remove from the oven and turn the broiler on with the rack 4-inches from the broiling element. Evenly sprinkle the cane sugar over each 1/2 plantain, then place under the broiler and broil until the sugar caramelizes. Transfer each plantain to the center of each of 4 warmed plates. Nestle a scoop of sorbet against each plantain and drizzle with the vanilla beet sauce over both.