It depends. In this case, a VBScript function is the way to go.
http://msdn.microsoft.com/en-us/library/3ca8tfek
For built-in script functions in Visual Build, see
http://www.kinook.com/VisBuildPro/Ma...temscripts.htm
For manipulating the Visual Build object model, see
http://www.kinook.com/VisBuildPro/Ma...bjectmodel.htm