Slice the potatoes crosswise into 1/2-inch pieces and place in a large mixing bowl with the blanched green beans. Dice the bacon across the strips into 1/4 to 1/2-inch wide pieces and add the bowl, along with the drained sliced pears; set aside.
To make the vinaigrette, combine 1 cup of the reserved pear juice in a mixing bowl with the vinegar, mustard, salt, pepper, garlic and onion powders. Slowly add the oil, whisking constantly until all of the oil has been added and the vinaigrette is emulsified. (Alternately, use a blender to emulsify the vinaigrette.)
To serve, drizzle the vinaigrette over the ingredients in the bowl and stir gently until everything is evenly coated with the dressing. Serve immediately, or hold chilled at 41°F or below until ready to serve. Just before serving, gently toss the salad to redistribute the vinaigrette and ingredients that may have settled to the bottom of the bowl.