Preheat oven to 350 degrees F.
In a large bowl, mix vinegar, cumin and chili powder. Slice potatoes into 1-inch size chunks. Toss potatoes with vinegar mixture. Let sit for 5 to 10 minutes.
Drizzle olive oil over potatoes and place them in a single layer in large rimmed baking pan or roasting dish. Season with salt and pepper. Roast, uncovered for 40 minutes, stirring occasionally.
Remove potatoes from oven, add onions and mix together. Return pan to oven and roast for an additional 20 minutes.
Serve warm with Argentinean Steak and Chimichurri Sauce, if desired.