A Visual Basic 6.0 application can be build with a "Setup Bootstrap for Visual Basic Setup Toolkit" installer. This has no available documentation on a silent unattended install process.

This setup typically consists of three files:

setup.exe
setup.lst
<application>.cab

The command line is:

setup.exe /s<log file>

For example:

setup.exe /sc:\setup.log

Note: The logfile must be specified, otherwise the error "Invalid command-line arguments. Unable to continue." is shown.

The silent setup will fail if the DefaultDir= parameter in setup.lst is incorrect. This will show as "Error determining allocation unit for drive" in the log file. Ensure the drive referenced exists.

Conflicts in file versions will stop and prompt to overwrite newer file versions. To resolve this delete conflicting files in the setup.lst file and renumber e.g.

[Setup1 Files]
File1=@MDAC_TYP.EXE,$(AppPath),,$(Shared),6/26/98 12:00:00 AM,8124720,4.71.1015.0
File2=@tx_pdf.dll,$(WinSysPath),,$(Shared),5/11/04 2:10:00 AM,471040,10.1.110.501
File3=@tx_css.dll,$(WinSysPath),,$(Shared),7/2/03 2:31:00 AM,356352,10.1.131.500
File4=@tx_xml.dll,$(WinSysPath),,$(Shared),7/2/03 2:21:00 AM,380928,10.1.121.500
File5=@MSVCRT.DLL,$(WinSysPath),,$(Shared),2/17/04 12:00:00 AM,278581,6.0.9782.0
File6=@mfc42.dll,$(WinSysPath),$(DLLSelfRegister),$(Shared),12/7/99 10:00:00 AM,995383,6.0.8665.0
File7=@tx_word.dll,$(WinSysPath),,$(Shared),4/28/03 3:10:00 AM,372736,10.1.210.500
File8=@tx_rtf32.dll,$(WinSysPath),,$(Shared),5/27/04 4:23:00 AM,159744,10.1.323.501
...

Becomes...

[Setup1 Files]
File1=@MDAC_TYP.EXE,$(AppPath),,$(Shared),6/26/98 12:00:00 AM,8124720,4.71.1015.0
File2=@tx_pdf.dll,$(WinSysPath),,$(Shared),5/11/04 2:10:00 AM,471040,10.1.110.501
File3=@tx_css.dll,$(WinSysPath),,$(Shared),7/2/03 2:31:00 AM,356352,10.1.131.500
File4=@tx_xml.dll,$(WinSysPath),,$(Shared),7/2/03 2:21:00 AM,380928,10.1.121.500
File5=@tx_word.dll,$(WinSysPath),,$(Shared),4/28/03 3:10:00 AM,372736,10.1.210.500
File6=@tx_rtf32.dll,$(WinSysPath),,$(Shared),5/27/04 4:23:00 AM,159744,10.1.323.501
...

Once these issues are resolved the install should complete silently using the above command line.

Manually Editing a Setup.lst File
http://msdn.microsoft.com/en-us/library/Aa263457

Format of the Bootstrap and Setup1 Files Sections
"Filex - A keyword that must appear at the beginning of each line. X is a sequence number, starting at 1 in each section and moving in ascending order. You cannot skip values."
http://msdn.microsoft.com/en-us/library/aa262320.aspx