What's the Definition of "Vital National Interest"?

I write this with some trepidation, as I'm an Air Force reservist and don't want to be seen as being in opposition to our current National Security Strategy.  However, some legitimate questions follow:

What is really behind America's repeated intervention in other countries' arguments with each other or among themselves?  Is there truly a vital national interest in sticking our nose in Iraq's intramural affairs?  Should we be trying to establish democracy in Afghanistan, a "nation" which has never had it, doesn't appear to want it, and wouldn't know what it looked like if it got it?  I understand and agree with "taking out the Taliban", because they were actively supporting and protecting Al Quaeda, which openly seeks to destroy us.  Wasn't that enough?  Has "nation-building" in such circumstances ever been permanently effective? 
What were the reasons that supported our intervention in Bosnia and Kosovo?  The genocide was horrendous, but was it our responsibility to quell it?  If their neighbors didn't do anything about it, why did we?  And if we intervened there, why didn't we do anything when hundreds of thousands were (and are) being killed in Africa by Africans?  Rwanda?  Darfur?  I'm not saying we should go into these areas either, I just don't understand why Bosnia is a vital national interest and Darfur isn't.  If economic considerations are so important, Darfur's where the Chinese are cornering the oil and gas market, Bosnia's got nothing really to offer us.

There's a distinct and profound difference between isolationism and keeping our own garden.  Where our allies or our citizens are threatened at an existential level, we should not hesitate to use our vast military might to extinguish the threat.  However, that other countries don't have the same regard for freedom, liberty, individual rights and protections that America does isn't sufficient reason to commit "our lives, our fortunes and our sacred honor" to influence the outcome of their internal, parochial or regional struggles.  Being policeman and big brother (not in the Orwellian sense, but in the "quit picking on my little brother or I'll knock you silly" sense) is expensive, complicated and unappreciated by many of the beneficiaries of our protective efforts. 
"Vital national interest" (VNI) is the touchstone whereby the United States decides whether to use military force against an enemy.  Unfortunately, there's no standardized definition of VNI.  The White House in a December 1999 press release entitled, "A National Security Strategy for a New Century", defined "vital interests" as "those of broad, overriding importance to the survival, safety and vitality of our nation.  Among these are the physical security of our territory and that of our allies, the safety of our citizens, the economic well-being of our society, and the protection of our critical infrastructures - including energy, banking and finance, telecommunications, transportation, water systems and emergency services - from paralyzing attack.  We will do what we must to defend these interests, including, when necessary and appropriate, using our military might unilaterally and decisively."

So far, so good.  Here's the problem: the document goes on to say our strategy is one of engagement - "Our strategy is founded on continued U.S. engagement and leadership abroad.  The United States must lead abroad if we are to be secure at home.  We cannot lead abroad unless we devote the necessary resources to military, diplomatic, intelligence and other efforts.  We must be prepared and willing to use all appropriate instruments of national power to influence the actions of other states and non-state actors[emphasis mine], to provide global leadership, and to remain a reliable security partner for the community of nations that share our interests."  In laying out this strategy, there is no distinction made among "vital", "important" and "humanitarian and other" interests, as it relates to the use of US military power.  It seems to me that our civilian leadership gets out the big stick too soon, too often and too indiscriminately, and we foot the bill. 

I tend to adopt the view of Michael Roskin in his 1994 Strategic Studies Institute monograph, "National Interest: From Abstraction to Strategy" that "the concept of national interest still has utility, not as an objective fact but as a philosophical argument in favor of limiting the number of crusades a country may be inclined to undertake."  Roskin hits the nail on the head when he quotes Morganthau's observation that "secondary interests" have the power to "grow in the minds of statesmen until they seem to be vital".   Yes, the same striped-pants Foggy Bottom crowd that hates and looks down its nose at the military is quite happy to say, "send in the Marines!" when one of their precious initiatives is threatened.