Sadly then your Vista partition will become unbootable either way.
As its dependent on C to boot. When a dual boot is created boot files are placed in a single partition. Vista will have likely placed its boot files on C. Re-Imaging that will wipe them out.
You'll likely have to run the Windows Visa setup to recreate its boot files, and then run the Windows 7 setup to recreate the dual boot options.
This of course assuming the restore process does not wipe out the other Vista partition in the process. Otherwise you'll have to recreate it before trying to restore.
Either way I suggest you backup the entire drive if possible just to be on the safe side.
I don't know of any good free tools for this. I use Acronis True Image for my operations, though its not free.
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Phil AKA Vacunita
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.
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