Hi Yadevol, welcome to NST.
I take it you installed Vista to E (separate HDD) ? but you are still booting from the XP HDD.
If Vista could see XP when you installed it, It would have put its boot files on the XP partition (because XP was "system") and the dual boot would have happened automatically as part of the install.
If You installed Vista "blind", all of its boot files will be on the new HDD, and the XP HDD will still contain an XP MBR and boot sector.
Put the Vista HDD first in the BIOS boot sequence
Install EasyBCD 2.0 latest build on Vista
add an entry for XP
Accept the offer to auto-configure the XP boot.
(don't use 1.7 - none of the automatic code is included. You'll need to do a lot of manual configuration, no longer necessary)