It shouldn't. If vista is already installed GRUB will pick it up again. The only problem I have ever had with bootloaders is installing windows after linux. But, to be safe I would download and burn supergrub or some form of bootloader live cd incase something goes wrong.
I would go here and download the iso there, just incase something goes wrong. I've done several multiboots though, and never had an issue. The disks I linked too have solved all my problems with Grub in the past automagically. Just read the descriptions and they'll point you in the right direction.