This is my first post, but I have sucessfully installed JaS10.5.4 on my laptop with Vista intsalled already. luckly I had 2 partitons Vista will shrink your volume for you to create a second volume for OSX. I now use Darwin as the bootloader. It just works out of the box
I can read the ntfs partiton from OSX but not the other way round...but Windows has allways been bad at reading Apple disks
I suggest creating a complete backup of Vista first, which is easy with Ultimate or Enterprise as it has it built in.
Good luck.