Yes, but it depends on where the break is. The door flaps for the vents are vacuum actuated, so even if there is a loss of vacuum, the doors go in to defrost mode by default. This is like a fail safe mode to make sure you can see out of your window even when things go wrong. This is very likely the problem you are experiencing- no matter what mode you select, you only get defrost. You will need to inspect the vacuum reservoir for leaks and if it tests good you will need to trace the lines and do the same. If the break is inside the cabin you will have to open up part of your dash. However, when I had this problem in my old SportTrac, the line had got loose from the reservoir. I just reconnected it and it was fixed. The reservoir was located on the passenger side behind the headlight between the fender and the wheel liner. I imagine it is the same area on the 3rd Gens.
