Bring the orange juice to a simmer in a saucepan. Add the prawns and cook just until the prawns are cooked through, about 3 minutes. Drain the prawns, reserving the orange juice.
In a saute pan, add 1/4 cup of the sugar and the pineapple and cook over high heat for 30 seconds. Add the reserved orange juice and vanilla bean and cook for 3 minutes, until sugar is dissolved. Remove from the heat and let steep while you prepare the vinaigrette.
Whisk together the sherry vinegar, vanilla sugar, and Worcestershire sauce. Add in 2 tablespoons of the pineapple liquid. Slowly whisk in the olive oil. Season, to taste, with salt and pepper.
Place the greens on a platter. Remove the pineapple from the liquid and place on the greens. Top with the poached prawns. Drizzle with the vinaigrette.