That is because XP and Vista have Different kinds of Bootloaders.
Depending on how you installed them
XP 1st then Vista x64
Vista x64 1st then XP there are many guides available to dual boot XP and Vista.
heres two of them : dual_booting_xp_with_vista dualbooting_vista_and_xp
interesting, i never had to do all that before. i was able to just install right along side and call it good. Oh well, i'll get vista back on here at some point LOL