Nothing was techinically "replaced", but they booted in phases rather than specific files. This lead to adding bootmgr(Windows Boot Manager) which reads a registry type file that was not used in Windows XP.
Windows XP and its predecessors.
Windows PE
Windows Vista was technically the replacement for Windows Xp. However, it was so unpopular that many users simply waited for the release of Windows 7 to upgrade.
Backup and Restore Center
Yes there is, they are different operating systems. VIsta was very unstable, and was replaced by Windows 7 less than two years later.
Windows Boot Manager (bootmgr)File,Windows boot loader (winload.exe)File and the Boot Configuration Data (BCD) File
There is virtually no risk, as no formatting or moving of files is necessary to perform the upgrade. Only certain system components are replaced.
That edition of Windows Vista does not exist. Windows Vista exists in two editions for home users: Windows Vista Home Basic and Windows Vista Home Premium.
Windows vista requires 512mb of system memory, an 800 mghz processor, 20gb of free hard drive space, and a direct X 9 capable graphics card. Windows vista is the 3rd newest version of Microsoft's OS systems, being replaced only by Windows 7 and 8.
No your windows 7 will not deleted
No. Obviously, if you upgrade to Windows Vista, you will have Windows Vista.
No.