Vibrations (aka wobble) is almost always attributed to the steering stem bearings. most likely noticed after you have put several thousand miles on any bike; even if the stem bearings were torqued down properly from the factory. The bearings wear will wear within their races and on further extended operation of the bike. As they seat and time elapses.. wobble can develop (play between the bearings and the races, exacerbated by forces pushing and pulling on that assembly which will exponentially transmit the misalignment to the handlebars); most especially noticeable when de-accelerating down through the 40-30 mph range. "wop-wop" will go the handlebars if you take your hands off of them.

Opas_Ride will give you a if you do that though... from past conversation with him. I have a memory like an elephant

Many manuals recommend retorquing the stem nut after a period of time. Because of wear. Of course we ignore that, as well as not replacing the spark plugs or checking valve lash on these motors. becase they are quite literally; bullet proof as empirical evidence gleaned through the ages of the GL1800 platform....

Had the issue on my past VTX1300R and replaced the bearings myself. Torqued them properly. No issue after that.

I'd venture that if you tightened down the stem bearing nut just a tiny bit; the wobble would go away. Of course it's involved though with the GL1800/F6B platform.

Steering wobble is VERY common with motorcycles. It's not fatal. But it is annoying. And can be remedied.