to start with you need to determine if the vibration is related to the engine or is coming from another source. As I mentioned previously try driving the car until the concern appears, then press the clutch and allow the engine speed to drop, if the car is still at the same speed and the vibration has gone this would confirm that the concern is from the engine.
If this is the case then have all the engine and exhaust mountings checked to make sure the rubbers are OK as if any rubbers fail this would result in all vibrations from the engine been transfered into the car body.
Check that the dual mass flywheel hasn't failed as this is quite common on this engine and results in harsh engine vibration, you can usually feel this more when resting your foot on the clutch pedal, as you press the clutch down slowly a rattle may be heard as well, another sign of flywheel failure is either grease or metallic particles in the starter motor, if the flywheel is suspected it may be worth checking the starter motor to confirm a flywheel fault. If the flywheel has failed then this means gearbox removal to replace it.
If the vibration is still evident when the clutch is pressed and the car is moving at the speed when the fault is evident then the concern is more likely to be steering or suspension related. Have the suspension joints checked, also check all the wheels and tyres to make sure they are still running true and aren't out of shape. Check the drive shaft joints especially the inner joints as wear in these results in vibrations as the vehicle speed increases.
If an injector were causing the concern I would expect this to be reproduced when the car is stationary, I wouldn't really expect a turbo fault to cause a vibration and again if this were the case you should be able to reproduce the concern with the vehicle stationary.
Start with these points, let me know if I can assist further or if you have more questions regarding this and I'll reply as soon as I'm able to.