An inflight failure of the airplane's only operating vacuum pump, which resulted in the loss of attitude information provided by vacuum-driven flight instruments. Also causal was the pilots' failure to maintain control of the airplane while operating in instrument flight rules (IFR) conditions, likely due to spatial disorientation, following the failure of the vacuum pump. Contributing to the accident was the operator's decision to dispatch the airplane with a known inoperative vacuum pump into IFR conditions.