I think you are having issues with the Vista Boot Loader. It no longer uses a "Master Boot Record" (MBR) as with Windows XP or Win2K3.
Try reading up on the following links about a free app called "EasyBCD":
Download EasyBCD at the bottom of the page: http://neosmart.net/dl.php?id=1
Documentation on what it is and its uses: http://neosmart.net/wiki/display/EBCD/E ... A465CDC6D0
Linux specific documentation: http://neosmart.net/wiki/display/EBCD/Linux
One issue you'll find a bit more complex is creating a partition large enough for your linux install, which depends on if you are using either an additional harddrive (which should be quite straight forward),
or if you are trying to install on the same drive with Vista installed by an OEM (where you might be dealing with a recovery partition and other hidden partitions). Vista can shrink partitions,
but you need to be careful with the order of partitions and may need to create an "extended" partition.
There are a number of helpful sites referring to gparted. But it probably is easiest installing Vista from scratch on a clean HDD, and define the partition size for Vista (download and install "EasyBCD" at this point), then install CentOS, making sure you use the advanced settings for where to place the grub loader. Next boot into Vista again and use the installed EasyBCD app. Instructions are in the above links.
Hope that helps...