lucy24 - 3:16 am on May 3, 2013 (gmt 0)
Under "Dynamically serving different HTML on the same URL", the article then goes on to highlight the two important implications of the Vary HTTP header, which are that...
If you are redirecting, the URLs are by definition* different. The "Vary" header is only important to search engines when you're serving different content from the same URL. That means the written HTML, not what the user's eyeballs see.
In htaccess, headers look like this (here quoting the popular "don't index my bleepin' robots.txt" header):
Header set X-Robots-Tag "noindex"
Or, in apache-speak-- with the parts I actually used shown in boldface--
Header [condition] set|append|merge|add|unset|echo|edit header [value] [replacement] [early|env=[!]variable]
"header" (the second occurrence) is the name of the header, here "Vary". "value" is what you want the header to say; this part is optional. The obligatory parts are only #1 what you want to do with the header-- set, unset, add etc-- and #2 which header you're talking about.
* By definition of the word "redirect", that is.