Unsure of what you are trying to accomplish here?
I assume on C: you have installed some other version of Windows other than Vista? It seems you are asking how in that OS to 'call' Vista and start it? Do you mean complete replacement of the running OS or run Vista under the operating OS?
Normally, one would install 2 OS's, one in C: and the other in another Partition or physical drive. Then, using the DUAL BOOT process, at boot time, choose which OS one wanted to run.
Here are a few articles to help you do DUAL BOOT :
Dual Boot info from MS
Another one
Note, that if the OS you are not running CANNOT read NTFS files, you will not see any files contained on that partition.
Irv S.