We've covered this before, so do a Search! Some use cheaper units but cover them in some way if it rains, but I own a bike unit that is weather proof. Living in the UK and travelling in Europe we have to be weather-resistant!
I use a garman 255 w with no problem with vibration. It works well if it rains I toss a ziplock sandwich bag on it not a problem. I think for it's a cost effective wAy to get a gps unit for a bike
You can adapt them with a mount from RAM. I have a zumo but to be honest with you other than the waterproof issue I can't see how Garmin can justify charging what they do for the line. I would think that you would be fine with a standard model and a plastic bag. You can buy a lot of replacements for what one zumo goes for.
I use a Garmin Zumo 665.....too much, now that I have it. Works great and i like the motorcycle or car feature. However, I had a Garmin 76CX hand held i used for hunting and in my auto before i got this one. It has a bean bag mount and i would use it on my Yamaha Roadstar Silverado and lash it to the tripple tree with bungee cord. Worked great. I'd recommend Garmin. They are tough units. I ran over my 76CX with my truck, and cracked the LCD screen so only 75% of the screen wasn't cracked or broke. I turned it on....still worked. I immediately ordered another to replace it. When it came time to order for the Ultra, no contest....Garmin all the way. jmho