Vista can not natively read Mac formatted volumes. Really the best bet is to have three partitions. This is what I use on my Windows machine for swapping between XP and Vista, but would work even better for your situation.
Partition 1: Mac OS X (Mac OS extended; Journaled): Use for OS, Apps and room for swap file and preferences
Partition 2: NTFS: Use for Vista OS, Apps, swap file, preferences and recovery console (if used, useless in my opinion)
Partition 3: FAT32: Use for storage of all files: I set up folders in the root directory for "Documents", "Pictures", "Music", "Videos" etc... and run aliases to my desktop or home folder in each OS and save to them to make easily available in each.
The main advantage of FAT32 is that both Windows and Mac OS X and read/write to the FAT32 file system, as well as just about any other OS except maybe some really old ones.
There are some disadvantages to this. There is a limit to individual file sizes of 4GB any file larger than this will not copy. The only time you may encounter this is with large videos, so if you use final cut/imovie leave room on the OS X partition. Another possibility is if you have a large .dmg of a DVD software installer, legitimate of course.
When formatting the partition Windows (XP and later) artificially limits the formatting size of FAT32 to 32GB, though FAT32 actually has much larger limits. However, they can make use of larger partitions you just have to do the formating in OS X.