Robert_Charlton - 4:18 am on May 3, 2013 (gmt 0)
I'm noting that the Redirection article doesn't get into the question of URL change and caching, as lucy does. Lucy's point about URL caching and the definition of redirection makes sense.
I have no axe to grind here... it may be that the second Google article glosses over that point, or that I've misworded the explanation, or it may be that Google has reason for wanting HTTP Vary Header to be used situation #3 as well... which does involve a URL change, and which is how I'm reading it.
In any case, I currently have no more time to give to the topic, but I'd love to see a resolution of the questions.