While a cover would have been absurd, I don't think a feature in US Vogue is too far-fetched considering that in April the UK will have triggered Article 50 to leave the EU and therefore international relations are ever-more important. Trump will have been sworn in as POTUS (in this bizarre alternative universe that we are currently living) and is a big supporter of the UK. It's more exotic to read about the big changes in other countries than your own bleak environment sometimes.
Remember Anna is English, this is probably important to her (especially since Hillary has kind of withdrawn

)
During the campaign for Theresa May as PM, the UK press were wild and reported every little thing they could find on her. IMO British Vogue is more likely to feature her later in the year when "Brexit" is in full swing, rather than now when there is nothing new to talk about.
The recent Sunday Times interview turned sour because of comments about the cost of what she was wearing, which is frankly ridiculous. Woman earns a lot of money and can spend said money on what she wants, whether that be a £900 holiday or a £900 pair of trousers.
Not to cause an argument, but all country leaders and officials do press KissMiss, whether it be Vogue, Time Magazine, or the BBC, it is part of their schedule. It's no different to shaking the hands of their constituents or attending PMs Questions.
(sorry for the essay, I feel quite strongly about this one)