To recover the SYstem MBR into Windows 7 partition, first mark it Active from Vista or the booted Windows 7 DVD Repair console or Repair CD: Partition - Mark as Active
Before you mark it active, check in Disk Management to see that no other partitions are marked Active. If so use the steps given above to mark the "INACTIVE" from booted Windows 7 DVD/Repair CD.
Now boot the Windows 7 DVD Repair console or Repair CD, click through to Recovery Tools list to run Startup Repair up to 3 separate times with reboots to write the MBR to Windows 7: Startup Repair
You cannot change the drive letter of Windows 7, however we can show you how to move it into the preferred lower HD address if that is possible after deleting Vista. Post back a screenshot of your full Disk Management drive map, using Snipping Tool in Start Menu.