My 2000 Mercury Sable engine lost power, engine vibrates when accelerating. Air always blow from defrost position. Cruise control is good.
The defrost is probably not your most urgent problem, so we’ll start with the engine.
A car that is down on power and shakes when it accelerates is not firing on all its cylinders. the trick is to figure out which cylinder(s) are failing to fire, and figure out why.
If you can remove your spark plugs, it should be obvious which cylinders are not working, that will narrow down your diagnostic to the known bad cylinders. You need to figure out if the spark plug is firing at all, or if you are not getting fuel to those cylinders, or if there is a massive leak letting your compression out.
He’s given no indication of tune. It could be time for ignition wires. That would give the same symptoms.
Normally I would ask of the defrost thing occurred at the onset of the misfire under acceleration, but a vacuum leak is unlikely if the cruise control is working …but that also begs the question …how does the cruise function (properly) if it misfires every time it accelerates?
I would start by looking around for a vacuum hose problem.
I agree with Joseph, defrost position is an indicator of a possible vacuum problem and that could very well cause the other symptoms.
I dismissed the the air flow problem as a secondary issue because the complaint is vibration during acceleration. Under acceleration, there is very little vacuum so vacuum leaks don’t much matter. Vacuum leaks show up and cause rough running at idle.
I agree w/ Manolito that this is unlikely a vacuum related problem.
xuefan2010, is your check engine light on? If so then you need to get and post the error codes. Many large chain auto parts stores read these for free. The format is P1234 - write down the exact codes and post them.
Have those spark plugs & wires ever been changed? If you verify that those are good you should have your fuel pressure and engine compression checked.
I’m in agreement about a vacuum problem but there could be a side issue to this; possibly an air leak in the intake tract that is affecting the MAF sensor.
Any vacuum operated dash controls run off of straight intake manifold vacuum and while it’s true manifold vacuum goes away during accleration, a vacuum canister and the one-way valve is supposed to take care of that issue.
Thanks for all your replies. I went to Autozone today and got a scan. It says cylinder 1 misfire. I checked spark plug it is okey and I replaced a coil at cylinder 1 (This car has six coils and each at it’s spark plug). The engine problem solved instantly. But, I still need to find out why air always comes out from defrost position whatever you turn it to any other positions.
Thanks for everyone! 
For the defrost problem, you probably have a vacuum problem in the control system behind the dash. On the back of your control switch is probably a plug full of vacuum lines coming off of it. At least one of these will go to a source of engine vacuum. The others will go out to control the doors in your ductwork. Something is either leaking, broken, or whatever. You’ll have to pull out the HVAC control unit.