The VRDB files are "virus recovery database" files. Avast makes copies of certain important system files, puts them in the chest for safe keeping; the process requires no input from the user, nor does involve any downloading. The files are copied from your own computer. You can set it to create a database when the computer is idle, or when the screensaver is running. Or you can disable it. (Not recommended; if one of the actual system files becomes corrupted Avast will not be able to replace it if a repair doesn't work.)
To modify: Right click the Avast icon in the system tray, mouse over "VRDB", the available settings, including an explanation, are there to see.
The updates you refer to as "program updates" are actually virus database, or definitions updates, and, as you have noted, are very small. This makes Avast ideal for dial up users.
Occasionally (there's just been one) the program itself updates. This update will be large, and on dial up, best set aside an hour or so for completion.
To modify how Avast updates, right click the tray icon, select "program settings", select "update basic" make the appropriate selection. I'd suggest having the database update automatically, and the program updates set to notify (Ask when update is available).