Put the potatoes, garlic and bay leaf in a saucepan; cover with water and season with salt. Bring to a boil, lower the heat and simmer until almost tender, about 5 minutes. Drain, discarding the bay leaf; transfer the potatoes to a bowl (halve any that are large). Add the peppers and scallions; drizzle with 2 tablespoons olive oil, season with salt and toss.
Preheat a grill to medium. Grill the vegetables, turning occasionally, until the potatoes are tender and the peppers and scallions are charred, 3 minutes for the scallions and 5 to 6 minutes for the potatoes and peppers. Transfer to a cutting board to cool slightly.
Scrape the skin off the peppers with a paring knife and cut them into large chunks; coarsely chop the scallions. Combine the potatoes, peppers and scallions in a serving bowl. Drizzle with the vinegar and the remaining 3 tablespoons olive oil. Sprinkle with the parsley and season with salt; toss.