Bro, this sound strange, Cultus has a reasonable A/C, I myself have one, though its on Petrol... I think you are having a problem of heated up car... the cooling system of the car seems to be the problem, Hotter is the car, lesser is the performance of the A/C...
You can do 2 things... get your radiator and Fan checked properly.. Get you A/C pulled out and get the lines "pressure flushed" properly.
On the other hand the RPM has to come down when you switch off your A/C, its doing apposite, this mean the Adlib switch is also not working properly...
The problem of having vibration before switching off mean that your car is Advance and most probably abnormally hot when you switch it off..
i'd say your problem is with the Car's engine Cooling system...