VRDB = Virus Recovery DataBase
It stores copies of certain legitimate files (Windows files).
If one or more of those original files get infected,
Avast will use the copies in the VRDB to replace the infected ones.
The VRDB is not (as David said) used to repair files.
He is however, correct with his explanation that only files that are infected with a true virus can be repaired.
For definations about viruses, malware, trojans and such, have a look
HEREIn order to see if the infected file is a false positive or not, submit the file to
JOTTI.
If other scanners don't detect it, it is either a false positive or a (till now) unknown (new) infection.