I have had a similar problem with my Mk2 Tdi since I bought it 3 years ago. The vibration is similar to a cylinder miss firing but which goes with road speed not revs and much worse with power on particularly on long hills. It once got very bad and my garage had a look at it and could find nothing identifiably wrong with the final drive and put it down to one tight cv joint that tightened up as the engine tipped on its mountings with power on. It was much worse when I had the gearbox reconditioned earlier this year but went back to normal after three - four days. As it is generally not a serious problem my garage has suggested I live with it so long as it does not get much worse and wait till the defect becomes serious enough that it can be isolated unless I wanted to take out a mortgage to investigate fully. Three years on still running and waiting for something to happen, the garage assures me it won't be catastrophic (Ihope). Hope this helps a bit but not and answer let me know how you get on