I have not done this myself, but perhaps these links may help to get you started:
How to automate Visio with Visual Basic .NET
I think you will have to do it via COM, and for this you will need the Visio PIA. The PIA is part of the Visio 2003 Software Development Kit (SDK). I would expect the SDK to have more guidance on how to use the PIA.
The SDK can be downloaded here:
You may also find this useful - it's about about Automating Visio Diagrams with a CSV file