Wouldn't a vacuum leak cause a lean condition?
Yes. According to the original post, it's barely running, which happened suddenly. Sucking air and oil through a leak between the intake and the head will oil foul plugs quickly, and make the engine run rough. Fuel fouling and sooty plugs from running rich, on only one bank is highly unlikely. Sooty plugs indicate a fuel problem. Black, oily deposits are a different can of worms. We simply don't have enough accurate information to do more than guess about this situation.
Jerry
