I have a server with various phpBB installation on it, so to manage the extensions easier I have set a folder containing all the extensions and in every phpBB installation I'm pointing to the needed ones with a symlink ("ln -s /full/path/vendor vendor" inside /ext).
This works ok except for INCLUDECSS and INCLUDEJS in template files: the url returned is a full path starting with "/server/path/phpbb/ext/" instead of the correct one.

I tried with

<!-- INCLUDECSS extension.css -->

<!-- INCLUDECSS ../theme/extension.css -->

<!-- INCLUDECSS @vendor_name/extension.css -->

<!-- INCLUDEJS extension.js -->

<!-- INCLUDEJS ../template/extension.js -->

<!-- INCLUDEJS @vendor_name/extension.js -->

but the problem is the same.