Install EasyBCD in Vista and add EasyLDR entries for the other OSes:
All you need to do is uncheck the checkbox labeled "Automatically detect the correct drive". Subsequently, a drop-down box containing a list of all mounted partitions will appear, and you can choose the Windows XP installation you wish to boot into. Clicking "Add Entry" will cause EasyBCD to automatically create the easyldr#, ntdetect.com, and ebcd.00x files needed to boot into the selected partition. You can then repeat these steps for as many separate legacy NT installs as you have available.
EasyLDR is required instead of simply adding NTLDR entries to prevent a 2 level boot menu. This is what the situation is like at present:
This is what is required: