Assuming your gearbox is ok, engine mounts ok, subframe all bolted up ok, I'd basically say that we want to get your engine running smoother then, and a drop in mpg also inditactes something is not as it should be. Parts can clog up but still work, hence no warning light or error code. I personally would do this:
Replace fuel and air filter
Clean MAF sensor and area around it. (Maf sensor is on top of the air filter box inside the start or the pipe)
Check turbo air pipes are not full of oil (turbo leaking oil/ turbo core worn out but you should notice strange noises from the turbo if this was the case and a loss in engine oil level)
Check breather pipes and little pipes to pressure sensors etc are not clogged
Replace CO2 sensor
Use a fuel system/injector additive in the fuel
If that cures it, should help your engine run smoother and give it more tork to prevent the vibration / engine struggle
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