You access these values via script, you would pass the values through to the child build like so in the VisBuildPro Project action's additional options field (on the Parameters tab):
/options "[Application.Options.Name]" /macros
"[Application.Macros(vbldMacroGlobal).Name]" /steps
"[Application.Project.Steps(vbldStepGlobalSubroutine ).Name]" /script
"[Application.Scripts(vbldScriptGlobal).Name]"
Another option would be to bootstrap the process by setting the registry value "HKEY_LOCAL_MACHINE\SOFTWARE\Kinook Software\Visual Build Professional\ConfigFilesPath" to the path containing your config files (this could be accomplished by a two-step VBP project to set the registry value and then chain to the main .bld project). Then the values would not have to be passed on the command-line at all. See
http://www.kinook.com/Forum/showthread.php?threadid=36 for more details.
This would require that all your config files be in the same location.