First you will have to describe your present partitioning scheme and if this will be on one drive or spread to another. When installing Vista after XP is already on you select another partition or drive for the new version to be installed to. During the installation the Vista installed will add XP in as a boot option under "previous versions of Windows" when you first start up.
For adding XP into the Vista BCD a free tool called EasyBCD can do this for you once you unhide the protected boot files and copy them to the Vista partition. For that go to the menu bar>tools>folder options>view tab to move the check to show all hidden files and folders following by unchecking the "hide extensions for known file types" and "hide protected operating system files(recommended)" boxes seen there.
Once the boot.ini file along with the ntldr and ntdetect.com files are visible and copied over to the Vista C root directory you then may need to edit the boot.ini to make a simple change there pointing to the XP partition. That usually sees the read only box checked in the right click properties screen and needs to be changed first.
The BCD editor included in Vista is the old method of manual configuration while the free EasyBCD tool will make life a bit easier for you there. You get that at http://neosmart.net/dl.php?id=1