This is not easy because your Vista is sitting on the outer boot sector of the disk which is needed to boot up the computer. But it can be done, although you need to use Windows 7 Backup Imaging now to make an image of your entire HDD to save externally so it can be easily reimaged in 15 minutes if this fails, which happens about 30% of the time in this case. Also back up your files separately to external.
You need to delete your C:Vista partition using 3rd party disk management software like free Easeus, or Paragon Partition Manager. I use Paragon from boot, but Easeus will run from desktop and go into boot to finish the job. Right click on Vista partition and have it delete it, then before Applying the changes, right click on the Win7 partition and have it Move Win7 over onto Vista's space using the slider so that it shows Win7 occupying all of the disk on the map. Then right click on the Win7 again and mark active. Click Apply all 3 changes and it will finish in about an hour.
When finished, it will likely not boot since there is now an OS sitting on the boot sector which has not yet claimed the boot. THis is fixed by booting from the Win7 Install disk and choosing Repair My Computer>Startup Repair. It may need to be run several times, as it normally won't take on the first try especially if it offers it up to you at first.
After it boots in WIn7, run chkdsk by right clicking on the Win7 drive in Computer, selecting Properties>Tools>Check for errors>select both boxes and schedule a scan for next boot, reboot and let it sort out any file system errors.
If this fails at any step, boot from your Win7 disk, select from the Repair menu>Recover Using An Image , wait for it to discover your saved image in the root of your external drive and it will reimage your HDD flawlessly.
