The Vitamin A story is complicated and Peter Aaby has been chewing away at it for many years. Sometimes Vitamin A supplements reduce mortality and sometimes they don't: or more precisely, in some places Vitamin A reduces mortality and in others it doesn't. Here is Aaby reflecting about what might be going on.
It's also a cautionary tale about generalisation: just because it works there, doesn't mean it works here. You have to think really hard about why it works there.
A catastrophic failure of generalisation followed the finding that premature infants in intensive care should be nursed on their tummies (prone); generalising that to ALL babies resulted in more cot death. Hence the 'Back to sleep' campaigns.