Those mirrors are a crap design, typically Ducati doesn't seem to concerned about what is behind you!
Guys on the Supersport 939 forum have several threads running about this with various fixes, most common seems to be a couple pieces of self adhesive black draught excluder/weather strip foam stuck under opposite corners or the mirror once you've got it adjusted to your liking.
I did this to a mates Supersport and it eliminated a good 80% + of the vibes. Some bikes seem to be worse than others, on some it's just the mirror class that vibrates on others it's the whole mirror/stalk assembly. I believe Ducati even replaced some of the worst ones under warranty.