My 91 Ford Mustang seems to be burning transmission fluid.

vacum module on trans bad

A transmission is not like an engine where it can "burn" oil like in a combustion process of an engine. The transmission can leak oil or have the oil drawn into the engine if the transmission uses a vacuum modulator valve. An engine should not burn much oil at all a quart between oil changes would be acceptable. For an engine to require excessive amounts of engine oil between services means the engine is either burning oil or leaking oil. Leaking oil should be pretty obvious, look for leaks underneath the engine at the oil filter, oil drain plug, and oil filter. Look for oil leaking from the timing belt cover area/timing chain cover and valve cover gasket. If an engine is "burning oil" look at the tail pipe and rear bumper for signs off discoloration or black oily deposits oil. Oil may leak down the valve guides particularly after the engine has been shut off and started from cold,if so blue oil smoke may be emitted from the tail pipe on start up, (this may also be noticeable if the car is left idle for a few minutes and then accelerated away for example after sitting at a traffic light). If the piston oil control rings are worn the engine normally smokes on acceleration. Oil control rings may fail due to an engine that has badly overheated or has had infrequent engine oil changes. Frequent oil changes with good quality oil is critical to long engine life. Perhaps changing oil viscocity to the next "thicker" grade would help.