This is the key statement from the DaveReid link:
These scalar units are uniquely tailored for each engine type to match the operating speeds and vibration characteristics of the engine rotors.
In short, there are engineering units associated with the vibe measurement, but that's not what gets displayed. The bottom line in what's displayed is:
1.0 or less is real good
4.0 or greater is real bad
On the newer installations (787, 747-8, and I presume the same is being done on the 737MAX and the 777X) the pointer pegs at 5.0 units, but the digital display goes to 9.9. The digital display goes "inverse video" when it reaches 5.0 to help get the crew's attention (instead of white numbers on a black background, the background turns white with black numbers).
With the notable exception of some Rolls installations, there is no Boeing or engine company requirement to shut down an engine with high vibes - rather the vibes are simply considered another piece of data for the crew to decide what action to take.