My book, Designing Visual Language: Strategies for Professional Communicators by Kostelnick & Roberts, is a technical writing book that I really hope to use next semester in Technical Writing.
It seems they define visual rhetoric as using vision to make sense of the world defined especially by design.
They want students to use visuals to get their audience to better understand what the text says. They offer throughout the book everyday career situations in which writers will be called upon to create effective documents using visuals. The exercises reallly help tech writing students to prepare for real-world situations in which they will really have to think about what they want to say and how they can best say it using visuals. For example, say your boss is cutting costs across the board, but you see the need for an expensive laser printer for the office. How would you (the tech writer) most effectively use visuals to persuade him to purchase the copier?