This link may help you out. You can use the ntldr to chainload linux just like you did in XP. There isn't a boot.ini file by default, but you can create one and vista will use it. So you can follow the instructions on this link:
When you install Ubuntu, create the boot loader on the mbr of the 2nd disk or on your root partition (/) of the second disk. Also, don't format the disk before the installation, so that Vista will ignore it for the present. Let the installer do the partitioning and formating.
If you have had only one disk, it would be best to let vista reduce the size of the disk. Unfortunately Vista is very greedy for disk space. I just got a new laptop with vista pre-installed. I don't see many of the problems others are complaining about (like UAC) but when I went to reduce the size of the 200GB disk, Vista could only spare 80GB!. It might be very optimistic about restore points, maybe.