I doubt that avast would allow the VRDB to become contaminated (that would definitely be a flaw in the process), I presume files are first scanned before data is stored to use later. Not all files are monitored/protected by the VRDB usually system files, .exe, .dll, etc.
If the worst came to the worst you could delete the VRDB file, C:\Program Files\Alwil Software\Avast4\DATA\integ\avast.int where the data is stored.
However, I feel you may be going to far in the what if scenario. How would you discover you have a virus in a file that the VRDB Monitors/Protects (you can't). As has been said before the VRDB has three entries (once you have run it at least three times) to try to recover from this very unlikely event.
One other thing to ensure this doesn't happen to me, I run the VRDB manually only after I have done an avast scan.