By default, NMV3270I selects the User Services menu as the initial menu selection. This can be modified on the Open call. However, no matter what function gets control and issues the first I/O, it is the responsibility of the program calling the interface and the NCL procedure (or other function) to agree to some protocol and maintain proper synchronization.
The Virtual 3270 Interface provides services which simplify VTAM session communication, and let you analyze the data being passed across the session using a 3270 full-screen conversation.