There have been several newsgroup postings about using VSTO to create Visio Add-ins. The deployment of these add-ins can be especially challenging. There is a new
developer article by Chris Castillo detailing how to write VSTO add-ins for Visio. Chris explains why using VSTO is desirable and aggregates information from a number of sources to walk you through the development and deployment process.
Creating Visio Add-ins with VSTO 2005 SE.