Defrost is the default position for low or no vacuum, most typical problem is in the check valve either leaking in the reverse position, or its hose to the intake manifold has a tiny crack in it as that manifold does get hot. Just cut off about a half an inch off of it and plug it back on.
It feeds the reservoir that effectively stores the vacuum, need that, when you open your throttle, intake manifold vacuum drops to nothing, so this maintains the vacuum for the MVAC system. Sometimes they have a small crack in the case, a piece of plastic, can be repaired with hot glue.
The is the vacuum source, a really nasty problem is have an intake valve that is not fully closing killing the intake manifold vacuum, easy to check with a vacuum gauge hooked up to that nipple the check valve connects to, should read around 20"/Hg at idle if at sea level. This drops an inch for about each thousand feet you are above sea level.
A more trouble some problem is the grease in the vacuum mode switch dries up causing a leak, haven't found one yet I couldn't take apart, clean, and use new silicone grease. Normally cost a small fortune to replace as they want to sell you an entire control panel. But some Fords have a separate mode switch, have to look that up.
Never had a defective actuator yet, but another possibility if the diaphragm is leaking. Vacuum circuit is extremely weak, so even the slightest leak can cure it. On some vehicles with an automatic parking brake release, that actuator can be the problem.
Would not waste my money on a Miti-Vac, cheap plastic, one drop of gas gets in, melts and seizes up, would only buy a metal one with replaceable O-Rings. Has a vacuum gauge installed and works like a vacuum gauge. Can just connect that to your intake manifold start the engine don't pump it and read your intake manifold vacuum. Can also connect it to your reservoir and pump it up to 20, after you reach that, should hold indefinitely.
Can also connect that to the hose that feeds the system, and pump it up to 20, that should hold as well. A necessary tool.