Answer
It is not possible to use VBA Script Code for the Calculate Field tool prior to 9.2. This has not been implemented into the tool. When VBA Script Code is necessary, the Field Calculator, by way of right-clicking on the field heading in the Attribute Table > Calculate Values, must be used.
When right-clicking on the field heading in the Attribute Table > Calculate Values, there is an Advanced checkbox that allows the use of VBA Script Code; however, no such checkbox is available for the Calculate Field tool in ArcToolbox > Data Management Tools.