What is the range of amplitude of vibration in a helicopter?
The amplitude of the vibration depends on the frequency and the acceleration: at a given acceleration, only at low frequencies do we get any significant displacement. If displacement is taken as the intensity indicator, the frequency plot is weighted towards low frequencies - the higher frequencies have a higher energy contents, that is why often a logarithmic relative scale is used (dB).

This document gives good information and background on measuring th severity of vibration. It can be done with an accelerometer: integrating acceleration yields velocity and displacement as well. From the document:


  Experience has shown that the overall RMS value of vibration velocity measured over the range 10 to 1000 Hz gives the best indication of a vibration's severity. A probable explanation is that a given velocity level corresponds to a given energy level so that vibration at low and high frequencies are equally weighted from a vibration energy point of view. In practice many machines have a reasonably flat velocity spectrum.


The vibration frequency plot in the previous answer is in dB acceleration, an earlier slide shows the reference. If it is displacement of a particular frequency in the plot you are after:


Read dB acceleration at the particular frequency
Convert into m/s$^2$ to get acceleration $a$
Amplitude = $\frac{a}{(2 \pi f)^2}$