In a blender or food processor, place onion and lemongrass then add the rest of the curry spices. Pulse several times to mix the spices into a paste. Scrape paste into a large bowl and add the lamb and potatoes. Mix well.
Heat the oil in a dutch oven or casserole and add the meat and potatoes. Using a wooden spoon, stir-fry the meat and potatoes for 6-8 minutes. Pour in the tomatoes along with liquid, water and vinegar. Bring to a boil and season with salt and pepper.
Simmer gently for 25-30 minutes, until the potatoes are cooked and the lamb is tender.