I set up a dual Boot of WIN98SE and XP Home years ago, Mate.
Vista, I believe only works on NTFS? Whereas XP Home and Pro work on either Fat 32 or NTFS.
Therein may lay your Problem? Particularly if you already have Vista onboard? Vista would automatically have used NTFS then formated accordingly.
Usually on a Dual Boot, you start with a Blank HDD, create Several Partitions, Format First Partitition as FAT32, usually called C: Drive and then leave the others alone. Install WIN98SE, Update and get it completely working.
After that it is Windows XP, or Vista's turn.
Windows XP can be installed on the next Partition whether it is FAT32 Formated or NFTS Formated and the same applies once again, It will become D: Drive with an option at Start up to Boot into either one.
Then you can either Boot into WIN98SE or WINXP.
Windows Vista is totally different and can only use NTFS, (As I believe) for the Example? Vista can now be installed onto the third Partition, same as the two previously installed ones at E: Drive and Updated as previously. But the difference is, Vista has no compatibility, really with FAT 32. (I must add that I am only working on Theroy now, not Practibly Knowledge. Others will know more than me on this.)
Now to Operate the System should be three choices, WIN98SE, WINXP, WINVISTA at Start up.
Whether WIN VISTA works with the other two, I will leave it up to more Learned Members than me, however I do know that XP will Work with 98SE, from experience.