Cut potatoes into quarters. Cut each piece into 1/2-inch-thick slices. Pour 3/4 cup broth into bottom of a 6-quart pressure cooker; layer the potatoes, green beans, and garlic in cooker. Close lid securely; bring to high pressure over high heat (about 9 minutes). Adjust heat to low or level needed to maintain high pressure; cook 3 minutes. Remove from heat; place pressure cooker under cold running water. Remove lid.
Drain potato mixture in a colander, reserving garlic cloves; place potato mixture in a large bowl. Combine reserved garlic cloves, vinegar, and remaining ingredients in a blender, and process until smooth. Drizzle the garlic dressing over potato mixture; toss gently to coat.