Q: I want to add application specific buttons to the toolbar of VPE's preview. Is this possible?
A: This is not possible. But you can embedd VPE's preview into a window of your own application. Doing so, you have two options to add application specific functionality:
1) Add a menu to your application's window - which has VPE's preview embedded - with the desired extra functionality.
2) Hide the toolbar of VPE and create your own toolbar within your application's window - which has VPE's preview embedded: all functionality of VPE's toolbar buttons can also be generated by calling functions of VPE. This way you simulate VPE's original toolbar and add desired extra buttons as required.