They didnt just disappear. You had Vista installed first. Then you installed XP. So Vista was not setup from the start to dual boot with XP. So in order to get them to work as a dual boot system you have to copy the boot files from XP over to Vista in order for the boot to work.
Now if you had installed XP first and then installed Vista that would have been done automatically. But since you did the opposite you had to manually do it. Cause the 2 systems use 2 different boot laoders.