I had troubles with vista boot loader and grub.
I found the solution to this problem
If you want to boot red hat entrprise with vista then you can install acronis os selector.
This acronis OS selctor allows you to select many OS on your PC at boot time.
It comes with acronis disk director suite.
If you don't want to pay for Acronis disk director suite then get the torrent file from some website.
Enjoy.