I'm not disagreeing, but aren't we de facto required to believe the visions of Catherine Laboure "I am the Immaculate Conception"?
We believe that Mary was conceived without the burden of original sin because that is a formal dogma defined by Pius IX, which followed not only the apparitions at Lourdes but also many centuries of universality, antiquity and consensus among Catholics. Pius IX formalized what was already widely believed.
Although I do believe in the apparitions at Lourdes and at Fatima and at LaSalette and many others, we, as Catholics. are not required to believe that Mary has ever appeared at any of those places or that she has ever delivered any messages or that she delivered the reported messages.
Thus, we may de facto be required to believe certain dogmas but that requirement itself is not the product of a required belief in any apparition.