Hello shashank646, welcome to Seven Forums!
The best way to get this accomplished to me would be to use Windows 7 disk management to delete all the partitions except the Windows 7 partition, they will then be unallocated space and retsart the PC to let it accept the changes.
First use the Partition Wizard Bootable Disk to resize the C: partition to the left to include that unallocated space back into the C: Windows 7 partition and restart the PC after those changes have been made.
Then use Windows disk management to
extend the C: partition to the right to reincorporate that space into the C: partition.
Once you have a single Windows 7 partition, use the PWBD to shrink/resize the right side of the C: partition to at least 50GB, that should be plenty for Vista to be installed to.
This tutorial at the link below will give you the basic idea how to use the PWBD.
Partition Wizard : Use the Bootable CD
Be sure to get the PWBD and not the installed version; scroll down to see the download link for the bootable CD ISO file at this link below and download the ISO file to the desktop.
Partition Wizard Free Bootable CD
Then use ImgBurn to burn the ISO to a CD, at no greater than 4x speed with a verify; it was designed for use and works best from a CD rather that a DVD
then boot the created CD to make the changes.
ImgBurn Free ISO Burning Software
Be sure to post back with any further questions you may have and to keep us informed.