Turns out that the Omnibus Reconciliation Act of 1981 did not explicitly "make ketchup a vegetable." That honor went to pickle relish:
[T]he FNS gave state agencies authority to explore additional alternates as long as they were reported to the FNS and met the meal crediting requirements. ... [A] state "could credit a condiment such as pickle relish as a vegetable (Section II, 5)".
Oh hang on. I just checked it. It’s the memory thing. So in my lifetime it was “pizza is a vegetable” from a 2011 bill doing the same shtick, making it easier for schools to say they are doing more when they are actually being given less.
A similar controversy arose in 2011, when Congress passed a bill prohibiting the USDA from increasing the amount of tomato paste required to constitute a vegetable; the bill allowed pizza with two tablespoons (30 mL) of tomato paste to qualify as a vegetable.