Once it was clear in 1965 that the South Vietnamese regime would fall without a massive increase in US involvement, LBJ would simply invoke the SEATO treaty itself (the "Manila Pact") as sufficient justification for sending more US troops and bombing North Vietnam even without any additional congressional authorization. (Though he might seek and receive such authorization after the decision had already been made.)
LBJ doesn't even
need another incident, real or exaggerated or fabricated. Arguing that North Vietnam's infiltration of South Vietnam and aid to/domination of the NLF constituted "aggression by means of armed attack" under Article Four of the Manila Pact
http://avalon.law.yale.edu/20th_century/usmu003.asp would actually be
less of a stretch than the legal justifications the US has used for some wars...