VRDB is a very important part of avast!.
Its size is something from 3-5 MB. In this database are stored "DNA" samples of all files,so if they are infected,they can be easily repaired to its original state. VRDB generating takes somehere around 15-30min,depending on your disk size and filenumber count.
avast! uses very little memory,so don't worry about running too much processes of avast!.