I'm guessing you must have also enabled "priority notifications, new in 5.0" -- AKA "heads up notifications".
Although not documented, these "heads up" notifications only work (i.e. show as "heads up") if an application requests vibration, so Aqua has to do this even if "vibration" is off in its notification settings...
Please check and turn off (and feel free to report this to Google, i.e. make it so that posting a priority notification doesn't require vibration).