Go into your Profiles (where you set your phone to normal, silent, vibrate, loud, etc). When the profile menu comes up, scroll to the bottom and select "Advanced..."
I'm assuming you want to edit your "normal" profile. Click the menu button and choose "edit" on the profile you want to change.
In this screen, select the email address you want to edit. You have two settings to update. Out of holster and In holster. In holster defaults to vibrate.
Hope this helps!