Dual boot Windows XP Prof (32bit) and Vista x64

Hello everyone. . . .

I am really lost here. I already searched the entire internet but i
couldnt find a solution. Maybe you can help me.

I want to install the two OS on one HDD, which i partitioned already
into 50/50 for XP and Vista.

I took easy path and installed windows xp Professional (32bit) first.
It runs fine.
Now i wanted to install vista x64 on the other partition (on the same
drive). The install works fine, all drives are detected and the files
are copied.
Then vista tells me that it needs to reboot (10secs).

However then, it just starts XP. No boot-manager shows up. I already
tried to point my boot.ini in xp to the 2nd partition on my drive but
then i just get an error that it couldnt detect ntldr. (so i guess
that is not the right way to go). (The files are on the HDD however)

Any ideas how i can motivate vista to continue its install and
register the boot-manager correctly so that i can choose between my 2
OS?

Thank you very much in advance.
Marvin