It may be a surprise but I am aware that a vacuum in a housing will help make a port lock! I don't think I suffer from a "classic lack of knowledge" and have been fortunate enough to have used most brands and types of housing (including some equipped with vacuum systems) more extensively than most.
Even with a vacuum in a housing, enough force will still move the port. I'll let other brighter persons do the physics, but 50 lbs quoted is actually about 22.5 kg. That is not a whole lot of force. Call me a Luddite, but I am much happier with a piece of metal that prevents the port from coming off. Of course, if a housing has both, I am also happy:)
The vacuum system did not detect the faulty bulkhead as the bulkhead failed catastrophically whilst underwater.
I think I should be clear that I do not rely on a "cautious test in the freshwater rinse tank" to ensure my housing's integrity, nor would I recommend anyone else to. Like many other active underwater photographers, I currently rely on a extensive maintenance and inspection regime before and after each and every time I open the housing. John, I would hope that, in the interests of balance, you will point out in your forthcoming article that, despite there being a lot of people diving with housings, the incidence of floods is actually fairly rare. This is because most underwater photographers are aware of the issues, and have strategies for dealing with them.
I think the point of my post was to point out that vacuum systems are a good addition, but should not be seen as a replacement for the current checks and procedures that most underwater photographers follow to prevent leaks. Sorry if I offended any evangelic sensibilities 