Season the brisket with salt and pepper. In a very large skillet, heat the oil until shimmering. Cook the brisket over moderately high heat, turning once, until browned on both sides, about 10 minutes total. Transfer to a slow cooker.
In a medium bowl, mix the ketchup with the brown sugar, chicken broth, onions and 1 tablespoon of the red wine vinegar. Pour the mixture over the brisket. Cover and cook on low for 8 hours, until the brisket is tender.
Transfer the brisket to a cutting board and let stand for 5 minutes. Skim the fat from the sauce. Slice the brisket across the grain 1/4 inch thick and return the meat to the sauce. Cover and cook on high for about 1 hour, until the meat is very tender. Transfer the brisket to a warm platter. Add the remaining 1 1/2 tablespoons of vinegar to the sauce and season with salt and pepper. Serve the brisket with the sauce and with buttered egg noodles.
The cooked brisket can be refrigerated in its sauce for up to 2 days before serving.