Place brisket in resealable food storage plastic bag or 3-quart casserole, cutting meat in half if necessary to fit. In medium bowl, combine vinegar, brown sugar, basil, salt and chili sauce; blend well. Pour over brisket; turn to coat. Seal bag or cover dish. Refrigerate 8 hours or overnight, turning once.
Heat oven to 325°F. Place beef and marinade in Dutch oven or 3-quart casserole. Place onions over top; cover. Bake at 325°F. for 2 hours, basting twice with marinade.
Remove cover; bake an additional 1 hour or until beef is tender, basting frequently. Let stand 10 minutes.
To serve, thinly slice across grain of beef; arrange on serving platter. With slotted spoon, place onions over beef. Discard cooking liquid.