> Not only is VirtualBox third party software but when Windows is running in VirtualBox it is also, by definition, third party software. 

True but that would mean EVERY question you can ask in http://superuser.com/ would be on topic as long as you include "virtual box" to the question. That is stretching it.

Installing virtualbox, installing tools for Virtualbox, how to set up Vbox is on topic. 

Installing an OS inside virtualbox generally is not. That requires knowledge about that specific OS we so not need to have. The same goes for the software inside a vbox. As soon as virtualbox does no longer matter for the question asked it is off topic unless it is about Ubuntu (so a question about Ubuntu inside a vbox is on topic).

> Similar questions about running Photoshop in VirtualBox have been asked many times in Ask Ubuntu without being flagged off-topic. https://askubuntu.com/search?q=photoshop+virtualbox

Sort from newest and you will see that almost all are about WINE. Not Vbox. And those about vbox are mostly asking about how to install vbox and are using photoshop as an example of what they want to do. I have yet to find a topic similar to yours.