Vaughn Spann’s “Now I Lay Me Down to Sleep” is a two-part arrangement: two series and one suite of works are on view at David Castillo Gallery, and three pieces are installed at the Miami River Armory, an enormous former hangar. The artist’s first foray into sculpture produced hulking monuments; in such a large space, they look spare and purposeful. For the young painter, a Yale graduate who had once intended to study science, this is a good thing: Assemblages of his work have been mistaken, he has said, for group shows. He veers between dense abstraction and figuration—and these days between