Maybe we have a problem with terminology. What do you mean by an "executable VI"? Do you have a plain VI (*.vi) that works properly and is "runnable" or did you turn it into an executable (*.exe) using the application builder?
Once you have made it into an executable using application builder, you could just use "System exec" (found in the "connectivity...libraries and executables" palette) to run it.
If you just want to call one VI from within another VI, you would just place it on the block diagram. Optionally, you would also need to assign terminals on the subVI so you can wire inputs and outputs as needed.