The brief
The Touran had power loss and heavy vibration under acceleration, with a rough idle, hesitation and a dip in fuel economy. That set of symptoms is a misfire. A misfire is a cylinder that isn't lighting cleanly. When that happens you lose the power that cylinder should give, the engine runs uneven so it vibrates and hesitates, it burns more fuel, and the computer flags it. The most common cause is worn spark plugs: the plugs light the air-fuel mixture in each cylinder, and as they wear the gap opens up and the spark gets weak until a cylinder starts missing. A scan for fault codes pins it down, but on an engine showing all of that the plugs are the first thing to check.
The diagnosis
A diagnostic scan confirmed the misfire and pointed at the ignition side. The plugs came out worn past spec, well into their replacement window, which explained it. The coils were checked and tested fine, so it was a plug-only job, no need to throw coils at it. So it was a straightforward fresh set of plugs to bring the combustion back to clean.
The work
Four new genuine VW-spec spark plugs went in, gapped to spec and torqued to the manual's figure. The coil packs were reseated on top with the harness clips clicked home, and the stored fault codes cleared. Then the engine was run at idle to confirm it was firing cleanly on all four with no stumble, followed by a road test to confirm the power was back and the vibration was gone.
The outcome
Power back, no vibration under acceleration, a steady idle, no misfires on the road, and the check engine light out after a drive cycle. The Touran went home running cleanly. Plugs are a wear item and once they're tired they drag the running down with them, so a fresh set was the cheap fix that sorted it.