I donāt think the term carries all that much legal weight in and of itself. The reason why the US did not officially declare war is because of US law and the dictates of congress. By keeping the Vietnam war as a police action, Johnson and Nixon were both free of congressional oversight. Certain laws come into effect once war has been declared (officially).
As far as escaping prosecution, exactly what do you mean by escaping? Who is going to enforce the charge of war crime should it be declared? The international court? You mean the one to which we do not sign on to?
The law is not justice. People always get those two words confused. There are proceedings against Cheney and Bush already. I believe Spanish law permits a charge of war crimes in abstentia (and maybe one of the Scandinavian countries too ? ). That only means that the named defendants cannot travel to Spain to āriskā arrest for these charges. Probably not even that given diplomatic immunity etc.