Only thing I can think of is this, however it is kind of hard to explain.
If you have ever had your dash cluster out and played around with the needle on the tacho, there is a point of resistance where the needle stops. This is at zero RPM. You can freely move the needle by hand but when you let it go, it will automatically fall back to zero. Maybe spring loaded or something. If you wind it to far forward there is another point of resistance. If you go passed this point of resistance it ends up changing the initial point of resistance.
Only reason I know this is because I have fitted white faced dials which means you have to take the needles off and put them back on again.
Could it possibly be that this spring loaded function is now broken and therefore the needle just falls to the bottom due to gravity and now doesn't want to move?
Test the tacho by changing the tacho sweep on the Vipec to different points: 2000, 4000, 6000 and see if it still sweeps to those points.
Quite strange never the less.