One item to note on your setup there is seeing Vista on a SCSI drive likely seeing a controller while two editions of XP are on ide drives. As a rule EIDE will override other drive types like Sata, RAID, SCSI. I ran into that on the last build here seeing two ide drives along with two sata HDs.
When Vista first came and I came across EasyBCD I had XP Home on the 1st ide, Vista replaced XP Pro on the second ide there, and then saw XP Pro on the first sata with the other strictly for storage. The multiboot worked by simply starting off seeing each installation set up as stand alone OSs and then proceeding to add the entrie in the Vista boot loader as well as the mbr copying the boot.ini and other XP boot files over and editing the boot.ini for each copy of XP.
Since you are using SCSI with or without a controller card you might be better off seeing Vista on the first ide drive followed by one edition of XP on the second and other on the current OSed SCSI. With two editions of XP installing Vista last would see those added into Vista's own boot loader where you would then see HnS and EasyBCD work.
Each copy of Windows being installed initially as stand alone will see itself as C not D or E when booted into that installation. In simple terms unplug one XP drive while installing the other edition and then allow the Vista installer to perform the detection and entries followed by seeing the two Vista tools put on after. The Vista drive would then be the primary host drive.