We need VIPs because our tanks are exposed to water too often and on occasion they are filled by incompetent dive shop operators (it only takes once).
Let me tell you a story:
About 30 years ago when I was working at Divers Service Center in Puerto Rico I bought three brand new steel 72 tanks. The very first weekend I used them, we had our tanks refilled at a dive shop in Fajardo, right on the marina. There were several of us unloading tanks from our boat directly to the fill station. The dive shop operator dropped the tanks into the cooling water (salt water) and filled them before I had a chance to blow some air to clear the valve.
That Monday back at DSC a gut feeling let me to open those tanks and VIP them. I am glad I did. Three drops of salt water, three pronounced lines of rust down the inside of the tanks.
A year later the inside of the tanks would have looked very ugly. Maybe ruined.
I remembered how nice they looked when I installed the valves the first time. Two weeks later I was tumbling them. It is amazing what salt water and high partial pressure of O2 can do to steel.
My point is (if it isn’t obvious) VIP’s do work. I have done my own at home and got the stickers from my local friendly dive shop.
I will get off my soap box now.
I do agree with all the other comments (I am not even going to comment about PADI). There is nothing wrong with 1/2” neck valves. They are used in oxygen tanks all over the country. Most portable O2 tanks use 1/2" valves.
I will try to find out about the legalities with bushings. I have been in touch with PSI (Professional Scuba Inspectors) many times this summer searching for information. I had three tanks condemned back in June by a local hydro station. They replaced them after I challenged their test data. That is a long story which I may post in a different thread.