The resolution is what gave Johnson the authority originally. That was the whole basis for everything and the language Congress used was pretty liberal. So to the extent that you are claiming its unauthorized/illegal whatever, you're wrong. To the extent that Congress agreed to give the Executive broad powers in SE Asia via t his resolution, it is hard to argue that Congress did not "declare" war when the President sought to use that power in certain venues and ways in SE Asia. This harkens back to Justice Jackson's concurrence in the old Youngstown Steel case, which has basically been adopted by most legal scholars....."When the President acts pursuant to an express or implied authorization of Congress, his authority is at its maximum, for it includes all that he possesses in his own right plus all that Congress can delegate." Ofcourse, Congress later wanted its power back via the 1973 War Powers Act (which in of itself is unconstitutional, albeit for a highly technical and correctable reason, although they haven't corrected it yet), although by that time the activities in Cambodia (and Vietnam) had been concluded from a US perspective, so that Act was moot.
You are blaming the wrong party. If you don't want the Executive (either Johnson or Nixon) taking actions in SE Asia that you feel are morally reprehensible, then perhaps the Congress should not have ceded its war authority to the President. You don't tell the POTUS to do whatever is necessary in SE Asia and then later b**** that you don't approve of some activity and claim its unconstitutional. There's a word I'm thinking of and it sounds alot like waiver.
"We may say that power to legislate for emergencies belongs in the hands of Congress, but only Congress itself can prevent power from slipping through its fingers."
ITS A PLAYOFF HOCKEY NIGHT IN PITTSBURGH!