why is my vectra in slow mode?

On a second thought, as you haven't said if the revs go up, if they do and the car doesn't answer, must be the clutch.
To test that, keep the hand brakes applied, press clutch pedal, engage first gear, accelerate and release clutch pedal. If the engine stalls, it means the clutch is fine. If engine keeps working with the car stationary and gear engaged, it means the clutch is slipping and it must be changed.

luics
February 2011

Looks like the throttle cable is broken or the pipes/filter clogged.
Does the engine increase rotation when in neutral?
Is it petrol or diesel?
If petrol, and the engine works, this means that the electrical part is working and so the engine is not getting enough petrol.
Could be the cable broken or the fuel pump defective or some clogging in the pipes or tank or in the gas filter.
1.Ask someone to press the throttle pedal while you look in the engine compartment if the movement gets to the other end in the carburettor
2. If so, then check the gas filter.
Please report back and if this does not work give more info. Next step should be testing the fuel pump.

luics
February 2011