You're asking for trouble, putting two OS's on a single HD.
It's much better to just use two HD's. I'm doing that now on three systems of my own.
But yes, you CAN repartition a Vista drive without damaging Vista.
I just did it a few days ago on a Vista drive of my own. I used "Partition Magic 8".
It worked like a charm. I created a "Storage" partition for misc. data and Ghost backup files.
First you have to reduce the size of the Vista partition, which leaves "Free Space".
Then you have to "Create" a new drive in the 'Free Space'.
Everything must be done in
precise order or you can wind up with a NON-Booting HD.
Proceed with Caution!
The Shadow