I did this job on a 1985 Jetta TD and followed the procedure in my Bently manual for the car. Head on the car. Basically do each cylinder intake/exhaust valve pair at a time with its piston at TDC (or valves will fall into the cylinder
) with the cam removed and lifters removed to allow removal of the valve springs and keepers. That means the timing belt has to come off, but I think it can just be loosened and slipped of the cam sprocket with minimal trouble and re-installed. Requires a valve-on-head valve spring compressor, valve seal removal pliers, and valve seal installation drift. I think these tools cost me about $200 ten years ago. Not sure if the procedure is the same for a VW TDI motor.
I'd make darn sure it was the valve seals first. I did mine because oil consumption was very high (a quart in 750 miles, car had 56,000 miles on it when I bought it) and it helped not one bit. The true problem was a broken compression ring in my case, creating lots of blowby purging oil through the breather.
Also make sure you did not get dirt into the motor running the draft tube (crankcase hose vent mod) I did that for a while in my '85 and got a grit into the motor. I think it was drawn past the seals from the vacuum created in the breather hose placed in the airstream underneath the car. This is expecially a possibility if you bypassed the pressure regulator in the CCV system. I hope this is not your problem and that it caused engine wear. I'd pull the valve cover and see if there is any grit in the head area. Good luck