When I had XP Pro, I had three partitions (1 for Windows and 2 for storage/backup). Then when I got Vista, I just used Disk Management to shrink one of the volumes (the second one) and installed Vista using the empty space.
The layout after doing that was like this: Windows XP | Storage Partition | Vista | The other storage partition
Yesterday I decided to remove XP but Vista's disk management wouldn't let me (because it is a system volume). So I booted to the Vista disk, formatted the XP partition, and overwrote the MBR (to remove XP as boot option). And BAM!!! Vista won't start anymore.
According to DISKPART, Vista is a logical volume sitting between the 2 storage partitions; I cannot mark it active, and I cannot make it primary.
If I delete - what used to be the XP partition - and attempt to use all that space for the Vista partition (using extend) it won't let me, saying that it cannot find any free space.
The only thing I can do is create a primary partition using the empty space, however, being empty it is of no use (I cannot boot).
I tried everything I could think of, including installing Linux (SuSE), but the installation won't mount Vista's partition because it doesn't see it as a bootable partition.
I would recommend booting from your Vista DVD and doing a repair installation. Vista was in your 3rd partition but it always puts the boot files in the first partition. So when you wiped your XP partition, the Vista boot files were also destroyed.