Other then it is the last OS installed that's bootloader is used. That is why you have to fix the boot partition if you do the install in reverse. Also you want to use the most recent bootloader as the older versions may not be compatible with the newer OS.
As far as I know the xp boot loader does not handle vista or win7, you need to use the vista/win7 bootloader for it to work, also you need to copy the boot.ini file from the xp install to the vista folder.
Anyhow here is some info on how to manualy edit the vista bootloader using bcdedit (comes with vista).
http://www.flexbeta.net/forums/index.php?showtopic=8373
longham if you like I can try to walk you through editing it manually if we can't get easybcd to work. But I will need to know exactly how your set up, meaning how many drives, on which partitions/drives/directories are the os's installed, what you have tried to do, which bootloader is running and which os is working and not working.