I'm glad that your enjoying the use of VisualRuby. As to working with Column Renderers, you will need to get the column first, which you can do by using the ViewCommon method: renderer(:id).
This will return the Actual Renderer for the column that you want to modify, or connect a signal to. So, you would do something like this:
tree = VR::TreeView.new(:x => String, :y => String, :z => String)
tree.renderer(:y).signal_connect("editing-started") do |*args|
..... do code here .....