I have a 100th aniversary  custom 1200 sportster, and since I bought it I have replaced both mirrors numerous times due to them breaking off where the thread stops due, I assume, to metal fatigue, and vibration. Is there some way to lessen the vibration.

odd... is it a efi rubber mount motor or solid mount? if its rubber mount then something is definitely not right.

Yes, solid mount. I've been told about soft mount kit for risers, but can't find anywhere.

Just found neoprene spacers on ebay.