Using a knife, carefully peel the grapefruits; be sure to remove all of the bitter white pith. Working over a bowl, cut in between the membranes to release the grapefruit sections into the bowl. Transfer the sections to a plate. Squeeze any extra grapefruit juice from the membranes into the bowl and stir in the vinegar and shallot, then whisk in the crème fraîche and oil. Season the grapefruit vinaigrette with salt and pepper.