I've got a Garmin Virb with their dedicated mount fitted on the top of the A-frame. There is a little bit of vibration but really nothing I consider detrimental to the footage. The wider the lens angle the less apparent it is too.
This was shot last week in slightly choppy conditions starting out at well over 40kts. If you watch the seat backs you can see the vibration induced relative to the A-frame. However, unless you go looking for it there is so much bouncing around anyway that it isn't noticeable IMV.
For some reason the first replay of this on Youtube just now came up fuzzy but was fine second time around. The stats on the right hand side should be pin-sharp.