Bob- State of Michigan requires an EGC in all flex regardles of length. There are two exceptions: One is Flexible Metal Tubing and the other is for flex 6 feet or less when used in one and two family housing subject to the Michigan Residential Code (IRC). Most contractors pull in an EGC all the time in flex regardless. I would say one of the primary reasons electricians run flex is to avoid running EMT or Conduit. The vibration issue is secondary.
