Decrease default vibration length on notification?

Automation is the preferred option. Many apps are available. I prefer using MacroDroid , since it is free (upto 5 macros) and easy to learn. To understand how MacroDroid works and the settings required for your Android version, see my answer

Note This is not tested on Lollipop as I am on KitKat but should work. Ensure appropriate settings for lollipop are made

Your macro would look like this

Trigger:

Incoming Notification → From →choose Application (in addition to application you can customise depending on the text content of notification)

Action:

• Vibrate →Short buzz (Ten other variations possible like Rapid, slow, increasing, decreasing etc)

Constraints: None

Edit: For more details and explanation on MacroDroid, please see my answer here How to prevent brightness from going to full when the battery low popup appears?