You mentioned vibration at a certain speed, but usually vibrations are engine related, and thus RPM related, rather than speed.
Speed related vibration ofrlten is called osscilation, or wobble, as it usually has to do with imbalanced tires, and/or the suspension making the experience of a wobbly, uneven road, worse.
I know sometimes I ride certain roads, and it feels like my wheel is out of alignment, because the road had been paved in a wavy surface, rather than a straight road.
Nothing much you can do, other than balance your tires.
Suspension issues are usually on older, worn suspensions, or broken ones.
I'm more interested in engine vibrations.
From the videos, I am pleasantly surprised to see that the dash cams give a quite clear image; meaning very little vibration compared to the Honda CB/CBR 300/500 F/X/R series.
Any engine has a certain Rpm range where they vibrate the bike more than on any other rpm range.
What I am experiencing is not a wobble or handling issue. It is sound. I don't know if the term harmonic resonance applies or not. Anyway 4th gear just sounds different, soon as you shift into it. You don't hear the sound difference so much under load if you are accelerating and going up through the gears...but if you are going 35 mph and decide to be in 4th gear and shift into 4th, there is the sound.
Someone else on the forum mentioned 3rd and 4th gear making a different sound. Problem is he didn't call it resonance and posted it somewhere else. Anyway, he said the mechanic told him it was a gear sound and should improve. I have almost 900 miles on my bike now and the resonance has improved/lessened. In part, that might be because I ride a lot in 3rd and 4th gear which is where I notice it. It probably is a gear sound.