It's not good engineering practice to preload those mount brackets, because stress promotes cracking and weld failures. It appears that there's a small amount of frame distortion from heat during the welding process, and this makes the mount location variable. I think it could be avoided by making the mount location and welding process the last operation done during frame manufacture.
The V is one of the smoothest parallel twins I've seen, and I was raised up on British twins and singles. Every bike will have an rpm range where there is a resonance where a certain amount of vibration will be felt. Twins are less critical than singles, and fours the least of all. My old K100 Beemer had a particularly annoying resonance at about 3500 rpm, and the solution was to not spend any more time than necessary at that rpm. The other alternative, of course, is to move on to something that one considers more pleasant to ride. I've done this in the past, and will do the same again if necessary. A bike that's unpleasant to ride doesn't get ridden.