Ok, this is what you CANNOT do with Vista. You cannot shrink it beyong a certain point if you have data on the HDD you want to partition.
So do the following. Grab the following
[urlhttp://gparted.sourceforge.net/]Gparted LIVE CD[/url]
Burn it, load it up. Select normal settings. It will boot you into the GPARTED LIVE CD.
From there, right click on your partition and click "resize". Choose the size you wish to change it to and voila. Click APPLY and let the process run. Should take a while as it has to copy everything and it does it very carefully. ( took me 6 hours partitioning 500GB into 2 ).
This method is guaranteed to work. Hope that helps.