Interesting topic. There was a school of thought that said it was the reduction of RPM that shed the ice however we have just been looking into this and both IAE and Rolls do not require the RPM to be reduced before take-off. Both say that it is the RPM (and therefore as mentioned before the high airflow) around the IGVs and centrifugal force of that RPM that will shed the fan ice. I would have thought that if there was still a high level of vibration after the runup, which would indicate fan ice and not IGV icing, then it would be a good idea to go back and have it checked.