I am sorry about this delay in responding. Please find my response provided below on your queries.
1. Would you please explain to me how the Printing Sample Zooming option works?
(a) While the Zoom tool is inaction, if you click on diagram, the Zoom Tool's ProcessMouseUp() method gets called and based on the button click(left button click or right button click), the Zoom In and Zoom Out operations are performed. This is done by setting the value to the Controller.View.Magnification property with the Zoom Tool class SetMagnification() method which gets called from the ProcessMouseUp() method.
2. Is there any VB.Net code that somebody wrote for zooming which can help me to start coding this feature?
I am afraid that, implementing this feature at the application level is somewhat a tedious process. However the basic idea for doing this would be customizing the select tool and set the desired value to the Controller.View.Magnification property within the custom select tools class ProcessMouseUp(0 method. Also you need to provide a suitable implementation in such a way that the diagram view gets scrolled to the zoomed are automatically. You can do this by setting the suitable value to the Controller.View.Origin property within the ProcessMouseUp() method.
I have created a simple sample that shows how to customize the select tool and how to set the Magnification property within the custom select tool's ProcessMouseUp() method. You can get the sample from the following link - http://websamples.syncfusion.com/samples/Diagram.Windows/F62013_June12/main.htm
P.S : You may need to implement your own code implementation in order to achieve the exact mouse based zooming functionality as mentioned in above statements.
I hope the above information will help you to get start with. Please look into the C:\Program Files\Syncfusion\Essential Studio\188.8.131.52\Windows\Diagram.Windows\Src\Tools\Zoom Tool class if you want to get any more details.
Thanks for your continued support to Syncfusion products.