Technical did an excellent thread explaining the ini files, and it's in there somewhere. Sorry I don't have a link to it handy, but I'm sure someone else will be happy to jump in and supply it.
With specific regard to VRDB update-frequency, you'll find you'll have to add a new line to the [VRDB] "paragraph" of the ini, since it's one of many settings where avast uses a default value (in this case 3 weeks) if there's no specific reference to it in the ini.