Let me get this straight, you want the StringVectorProperty to be initialized from the content of a file, which is identified in any another property ?
I’m afraid that is not possible in a Python plugin.
Afaik, it is not (yet) possible to initialize a property using the value of a single string in a FielData. (which could have been a way to emulate what your are looking for).
This would be doable only with a C++ plugin with custom widgets.