I strongly recommend Andrew Sprung's epic takedown of a particularly awful column by Gideon Rose in the Sunday NYT.
Sprung hits most of the important points, but there's one more worth making. The Nixon theory was that the danger for the US in leaving Vietnam and allowing the loss of South Vietnam to appear to be US policy would be that future allies and enemies alike wouldn't trust the US to stand by its friends.
But that's a dubious proposition to begin with if, as Nixon expected, South Vietnam was going to fall anyway.
Moreover, it's not the only possible lesson that enemies could draw from Nixon's strategy, which after all, as Sprung points out, was extremely costly to the US. The other lesson is what apparently bin Laden believed: that the US could easily be enticed into fighting long, drawn-out stalemates -- and that once in, it was fairly easy to keep bleeding it indefinitely. As long as you (that is, bin Laden or North Vietnam and it's allies) are willing to absorb the damage, too.
Generally, "we're easily duped into doing something stupid and self-damaging" isn't nearly as wise a message to send as Nixon, and Rose, seem to believe it is.