How can I reduce the length and intensity of SMS notification vibrations?
One option is to disable the notfications from the built-in messaging app (in the Messaging app, press Menu, select Settings and then scroll to the bottom and take the tick out of the Notifications box).

You can then use a 3rd party app that gives you full customization of the notification options, like SMS Popup which gives you control over the vibration duration and pattern, as well as a lot of other notification options.