Why is the mirror vibrating so bad I can't see behind me?

I had to replace all the screws holding my mirrors on after a trip to Iowa three years ago. They worked loose on all the great highways we have across the country. I used stainless screws, large fifty cent size flat washers and lock washers. Their was no support to mount nuts on the fiberglass cap so I made up some from a tube made of aluminum and drilled the appropriate sized holes thru them.