It is impossible to make a Vin dead smooth at all RPMs as this factor is a compromise to give you the smoothness at the most used RPM range. So, it stands to reason that there are going to be other RPMs not so smooth. Oddly, the machines I built also had
80 mph in the sweet spot. To me it does not sound like you have trouble. Check out what bothers you vibrating in tune with the frequency of that exact range and do a little adjusting or remounting of those components. Crazy, but handlebars and vibration frequency
can modified with Gorilla glue which weighs nothing.... plug on end.... wet the inside... and pour in a bunch of Gorilla Glue. it will expand a lot as it turns into foam..... let it do it's thing and cut off the mess at the open upper end when dry. Amazing.
Interesting!, how much Gorilla Glue do you need for a set of bars?