I would like to make a small modification to the script that generates the website domain directory structure. I simply want to add a sub-directory called "vendor" under the private directory. Could someone please point me the correct direction. Many thanks.
I think the correct way would be to create a server plugin that runs on your web server and hooks in to the website creation event and create that directory. Search for example in form posts here, it should be pretty simple. Another alternative would be a cron job that runs outside of ISPConfig and loops through all private directories and does the create.
The question is why you want to do that at the first place, as it is not really advisable to modify ISPConfig default files unless you really know what you are doing? Anyway, I really think no website software requires that even if its public folder and vendor folder are in parallel. You can simply point your website to use public as your website root and vendor folder will only be accessible privately. If that is what the software requires of course.
I know and what I mentioned is the easiest solution without you have to meddle with ISPConfig files. Even latest flarum (beta 8 development version) is almost comparable in its folder arrangement but is definitely installable using the same solution.