I had this same problem with Vista.  I was using the onboard audio on my dell (SoundMax), and i had no options to set line-in output volumes.  I went and purchased a $30 Creative Audigy SE card and installed the Creative Beta drivers and now I have options in the Vista Mixer to change line-in output volumes.  So it appears it is a driver issue and not neccesarily a Vista issue.  I do with the generic drivers that whipped with Vista had this support.