LBJ needed to finance the Viet Nam war and the Social Security trust fund was the answer. By moving it "on budget" in 1966 he made the funds available to all the congress critters to spend as they liked.
Nope. It was always "On Budget" whether they wanted to call it that or not. The money always disappeared into US Treasury Notes as soon as it was collected. Guess what the US treasury did with the money before 1966?