I own a w204 180K 2008 and recently I have noticed a mild vibration.
It happens on this exact case mostly. If I am driving at lets say 2k rpm, I put off my foot from gas to slow down until 1k rpm and then when I accelerate again slowly it starts to vibrate. Surprisingly when I accelerate hard it does not vibrate.
The vibration is felt through the cabine and under the seat mostly. The moment it passes 1,4k rpm the vibration is gone. It is pretty hard to reproduce the issue even for service shops since you have to drive the car this specific way to cause it. I already checked the transmision and the guy said it was fine but he also said that it might need fluid change and a new trans mount. Another mechanic said that it might be the vacuum pipe or ignition coils. Do you have any idea what could it be? Even the slightest clue would be appreciated.

Thank you

Sounds to me like the transmission is slipping when you get back on the gas. And when you get on it hard it'll downshift and you don't have the slipping.
Try to drive the car in manual mode and see if your shifting gets rid of the vibration. It's a easy first test

Transmission fluid FOR SURE.

Might as well do the mount it's definitely roached at 180k.

My car did this too & also swayed, which i later was told by multiple mechanics it was bc my alignments needed adjustments. Good luck!