VBA and VB have different entry points to the object model. The VB entry point is the application, whereas with VBA you are already in the application. Also, with VB you have to reference the AcadApplication.ActiveDocument property in the AutoCAD Automation API explicitly, but with VBA you can use the global "alias" for the ActiveDocument property, ThisDrawing.
For information about writing VB versus VBA code, click 
.