Partition formats for Vista

I think you mean FAT32? FAT32 is readable and writable by Vista and OS X, though there is a file size limit (4GB), so you can't put files on it that are bigger than 4GB each.

Under Windows you can only create partitions that are 32GB in size for FAT32, so if your drive is bigger you might run into problems there. However it looks like if you format it using a different program (or maybe under OS X if there is an option for it) you can get past that 32GB limit. Supposedly this limit is only there under Windows to try to get you to use NTFS instead, even though Windows will read >32GB size FAT32 partitions just fine despite the fact that you can't create them.

Sorry if that's a bit confusing. Say you have a 150GB drive and want to format it as one big FAT32 partition, you won't be able to under Windows unless you use a 3rd party program, or do it under another operating system.

The other format readable by both is NTFS. It has full read & write support under Vista/XP but only read support under OS X. However, there are 3rd party options (ntfs-3g) that let you write to the NTFS drive as well, though there might be some risk.

You could also format the drive as HFS+ (Mac format) and use an experimental driver for Windows that lets you read it (Macdrive?) but I haven't read much about that. Your best option is to do FAT32 for the whole drive if you don't plan on having files over 4GB, and want to read and write in both operating systems.