The Triumph ovals I have work great and have never vibrated. Seems to be more of a common problem with the tear drops.
Mick, your bike is just outside. I doubt too many of us have dismantle a stock mirror lately to be able to answer your question. You can, by the way, get adapters that will allow you to fit whatever you want. Some aftermarket mirrors even come with them. Not sure why you seem so pissed with all of us, or Triumph for that matter. If you don't like something on the bike, change it (like the rest of us do).