I had a Nuvi 2455 mounted to my unsupported accessory bar for at least 10,000 miles and never found that it vibrated too much to be usable. The mount I use is the standard RAM cradle and double socket short armature for the GPS, combined with a homemade accessory rail mount made from hardware store aluminum and a threaded ball from McMaster Carr. Once I added a homemade mount for my Droid Turbo to the accessory rail, I added those aluminum support brackets that were made by a member on this forum (Propsoto). The thread for those brackets is here:
http://www.yamahasupertenere.com/index.php?topic=19196.0
With those brackets added, the accessory bar is solid as a rock, and easily carries the load of both my GPS and phone without any vibration.
The photo showing the construction of the homemade ball mount is actually a photo of a ball mount I made for my handlebars. The concept is the same for the accessory bar, except I used only a single small U-bolt instead of two larger ones. The ball mounts are easy to make using basic hand tools (hacksaw, power drill, and files/sandpaper).
I like my GPS mounted to the rail. I don't have to move my whole head and take my eyes off the road to look at it while I'm riding, I just have to move my eyes down a bit so it's in my field of vision.
If you don't want to make your own ball mount, RAM makes a single ball mount that should work. I needed to make three of them, so it was cheaper for me to make them (I have lots of scrap aluminum, and the threaded balls were only a buck). Here is the ball mount from RAM:
https://www.amazon.com/Stainless-U-Bolt-1-Inch-Inches-Diameter/dp/B002AQV51A/ref=sr_1_1?ie=UTF8&qid=1479918816&sr=8-1&keywords=RAM+handlebar+ball+mount