There are essentially two types of geodatabase customizations you can model: Custom Features and Custom Object Class Extensions. Custom Features are developed to extend the behavior of the objects in a class, and Custom Class Extensions extend the feature/object class itself.
Aggregation is necessary for the creation of Custom Features. Aggregation is the process of inheriting code from another object, and it is necessary so you can inherit the default feature behavior from the ESRI-supplied feature object and then add your own behavior. The code generation wizard in Visual Studio has no option to create stub code because this language does not support aggregation.
Visual Basic can create Custom Class Extensions; however, because aggregation isn't necessary in this case. If you want to use Visual Basic to create a Custom Class Extension, you won't be able to use the code generation wizard because it can only generate C++ code. Class Extensions are customizations based on the feature/object class itself, such as custom renderers, custom validation, custom property inspectors, and other class level behaviors.