Did I say anything about caching? I think that was a different thread.
I was looking for the merits of what you were saying, Lucy, and caching was an implication I drew from this particular comment you made....
The "Vary" header is only important to search engines when you're serving different content from the same URL.
Google suggests that Vary Headers will alert ISPs etc who might otherwise serve cached pages to "consider the user agent when deciding whether to serve the page from cache or not." (Having the correct page served, of course, also indirectly helps Google.)
But, if the URL changes, as with the mobile subdomain added, there would be no reason for an ISP to serve the cached page. They would be dealing with a different URL. Thus, I felt that you might have had caching in mind (among other considerations) when you made your comment about the importance of the "Vary" header.