I had similar ages ago. Had the front tyre done and it wasn't balanced. I was getting a small vibration which started on the foot rest, then worked its way up to the tail end and the seat, Only happened at higher speeds like yours.

I only found out when it was in for a service and asked the yamaha mechanics to look at it. He managed to replicate it and narrow it down to the front. Bizarrely the front end was not vibrating at all which is why I failed to work out what it was myself.