Even though this involves my 2014 XL1200cp Sportster, I'm posting here because the question is of a general nature.

For example: when I feel vibration "peaks" at, for example, 2700 rpm, 3200 rpm etc., is that because the engine is actually creating 'extra' vibration at those rpm OR because at those rpm there is a sympathetic vibration in the frame/handlebars/forks etc.?

Expressed in another way . . . the mechanical parts of the bike are resonant at those rpm so the vibration felt is increased..

 

If the vibrations were strictly due to imbalance in the engine, the vibrations would build and keep building as the engine speed increased. What you are sensing is a resonant frequency with the entire assembly.

 

High frequency vibration in bikes that are more sophisticated than your Sportster is referred to as "Buzz". It's still out there.

 

When you start to feel it, shift.
No tachometer...saves money.

 

Should have felt the vibrations of a '99 Sportster 1200 with solid mounts. I swear the paint mixers at Lowes are smoother.

 

Yep ^this. Just like a good old American muscle V8. You feel the shift. you don't need a gauge to tell you.