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Vibrate On Ringer
Whether to vibrate on an incoming call.
You're probably really looking for the action Audio / Silent Mode if you want to put your phone in vibrate or silent mode.
Note: if silent/vibrate mode is engaged, this setting is overridden.
Note: if you are going to use Tasker for this, it's probably best not to mix it with other methods of controlling the setting.
See also: Ringer Volume.