Following is a chart from the Microsoft Support Lifecycle website that helps explain what is lost when Mainstream support ends:
In order to access updates on Windows Update, it is important to update to Service Pack 1 (SP1) which extends Mainstream Support for Windows Vista to July, 2011. Following SP1 installation, upgrade to Service Pack 2 (SP2 requires SP1 installation first). SP2 support is currently supported until April 2012. After that date, Extended Support is only available to Windows Vista Business/Enterprise to 2017.