Hi ulisse,
if I do have high reading on eng vib values during the run up (5 or 6 n1 vib)
You should wait until the vibs have returned to normal (within ??? secs whatever your manual says). As soon as the vibs are normal, you may assume the ice has been shed so can commence the take off.
We wouldn't take off if the the vibs were still abnormally high after 60 secs (RB211-22B time limit).