I admit that I've always had problems with the term "vital interest". It doesn't seem to be well-defined because apparently everyone American is free to use it in support of his opinion on security policy no matter how trivial the issue is.
I found this source which offers a quick overview:
National interest roots trace back to the Machiavelli era. Machiavelli’s concern was Italian unification and liberation from foreign occupiers. By the nineteenth century Clausewitz contended that all states are motivated by their need to survive and prosper. In the 20th century the seminal works of Hans Morgenthau considered only two interests exist: vital and secondary. Throughout the 20th Century, and most notably during the Cold War, a number of commissions established categories for compartmentalizing our national interests. The first real post-Cold War scrutiny of the compartmentalized interests occurred in July 1996 when the Commission on America’s National Interests established that there exists four levels of US national interests: vital; extremely important; just important; and less important interests.These interests look no different from those established prior to and during the Cold War (...)
I've still got difficulties to grasp the idea because it looks essentially arbitrary and illegitimate to me.
There are in my opinion only three categories of national interest:
(1) National interests that justify violence in their defence
(2) National interests that don't justify violence
(3) National interests that aren't understood
Category (1) was officially defined by signing and ratifying the Charter of the United Nations. Article 2 says it all.
A state who decides that the agreed to rules don't apply any more should make its outlaw status official by leaving the UN.
Category (2) may have several degrees. Some interests may justify great expenses while other barely justify that the ambassador talks to someone. These different degrees aren't of great interest to me.
Category (3) may of course fit into the other categories, but it's sufficiently different to consider it separately. It's mostly of interest to historians, though.
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Now have a look at this Gallup poll:
W T F ???
None of these issues ranks highly in category (2), much less (1) in my opinion. These issues are unpleasant, some of them hyped-up, but - honestly - there's almost open warfare between criminals and the state in a neighbouring country and this doesn't even make the list?
There's furthermore one very obvious threat to the national interest in every country, one which is usually being underestimated; the fallibility (or worse) of the own government (including parliament). I'd rate this higher than everything on the list.
(Gallup asked about the rating of "possible threats to the vital interest of the United States in the next 10 years".)"
The terminology for national interest "threats" seems to be FUBAR in English language.
Hat tip to greatpowerpolitics.com for the Gallup poll.