The brief
The Jetta had been losing power under acceleration, the engine vibrating enough to feel through the wheel under load, and there was a hesitation off the line with a rough idle to go with it. Eventually a check engine light came on. 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 shakes and stumbles, it burns more fuel, and the computer flags it. The parts behind it are the spark plugs, which wear out, and the ignition coils that sit on top of them, which fail with age. On an engine that's showing misfires, both are suspect, so it needs a proper diagnostic scan rather than guessing at one part.
The diagnosis
The scan confirmed the misfire and pointed at the ignition side. The plugs came out and were worn past spec, well into their replacement window. With the plugs that tired and a misfire logged, and the coils being the same age and the same wear item, the call was to refresh the whole ignition side, a fresh set of plugs and a fresh set of coils, rather than chase one cylinder at a time and be back soon for the rest.
The work
New genuine VW-spec spark plugs went in, gapped to the manual's figure, and new genuine VW-spec ignition coils went on top of them. The harness clips were reseated, and the stored fault codes cleared. Then the engine was run to confirm it was firing cleanly with no shudder, followed by a road test to confirm the power was back, the vibration under load was gone, and no misfires returned.
The outcome
Power back, no vibration under load, a steady idle, no misfires on the road, and the check engine light out after a drive cycle. The Jetta went home running cleanly. Plugs and coils are wear items that age together, so doing the full set in one go means even, reliable ignition across all the cylinders rather than another misfire a few thousand kilometres down the road.