Add file association by double click to existent MFC application

We have a relatively simple application (only one dialog widow) that contains the typical "File | Open..." menu item. We would like to add the option where the user double clicks a file of the corresponding type/extension (*.vtl) and next the registered application is located and executed. If I am not mistaken, the Windows OS passes the path of the double-clicked file in argv[1]. In fact, this feature USED to work in this app but with several coders maintaining it, one of them probably broke the code. Then again, perhaps it was in a wish list but never implemented. Note: We DO NOT hire programmers that do not have a Freelance/vWorker rating.