The bathroom wall of a rent-stabilized apartment on Manhattan’s Upper East Side, where a postal worker and librarian couple have lived since 1963, was decorated with a pencil drawing by the artist Sol LeWitt. Until a few years ago, when Herb and Dorothy Vogel donated the bulk of their 4,000 artworks to the National Gallery of Art, the bedroom walls were crowded with pieces by Joseph Beuys, Robert Mangold and Richard Tuttle. The documentary “Herb and Dorothy,” now on DVD, tells how the Vogels amassed a world-class abstract art collection...