is just one possibility but it is the one I prefer because any one system breaks down will have no effect on the other.
The standard approach and the easiest is to combined the two Windows, either let XP's NTLDR dual-boot Vista or using Vista's BCDedit to dual-boot XP, and use Grub to boot all. That approach has the disadvantage in that if one of the Windows in trouble it may kill both M$ systems or require the re-installation of both for sharing a common boot loader between them.
The above link allows the 3 systems intalled in any order. It is also suitable for installing any number of M$ systems and is the basis for installing 150+ systems if you have them.