Instead the sounds are directed through the mouth. This may be a useful task in trying to show a child why you are doing exercises. However, in a child with VPI, they can't help but have air come out of their nose with these words. They could try and feel an adult's nose who doesn't have VPI, but this may be too confusing for the child because they can't do it on themselves. Be sure the child understands that they may not be able to achieve no air coming from the nose.
Secondary surgery involving cleft palate is designed to improve velopharyngeal function.
Let the child feel the difference between nasal and mouth sounds by placing his fingers on the bridge of your nose. Be sure that you have experimented for yourself before you demonstrate for the child, and that you are producing these sounds correctly and directing them through the proper passages. Also, it is completely dependent on where you live. In some parts of the United States and in other languages, higher degrees of nasality are accepted as the norm. The difference is too slight to be able to feel via vibrations.