I have a 1967 T100R - Triumph Daytona and am experiencing license plate fractures at the license mounting holes.

I have mounted the plate with rubber grommets and have still have the failure / fractures

What have others done to prevent the vibration from fracturing license plate?

Yep, on my bikes I've cut backing plates out of sheet aluminum to keep the original-year plates from cracking.

Use the license plate as a template, cut the backing plate with a jig saw, drill the holes, finish the edges, polish, and Bob's yer old man's brother.