Given the amount of time it takes for LV to launch, you would probibly get better response time if you let the LV code run in a loop watching a predefined "input" directory for new files to process, and writing its results to a predefined "output" directory. The code that uses the text files would likewise sit in a loop waiting for a new text file to appear. (BTW: what will that other piece of code be written in?)
Alternately, if you don't like the idea of polling a directory, make the main LV application event-driven with a user event that performs the processing. Then create a small VI that you run from your C++ code that simply fires that user event.
Finally, given all the limitations of C++ perhaps the most straightforward solution might be to write the whole application in LV?
Mike...