The strange, wonderful thing about the documentary Finding Vivian Maier is how it uncovers one story, only to veer off and tell another one entirely. On one level, it’s about a young man who discovers the forgotten work of a genuine artist, then takes it upon himself to share her work. It’s also a character study of an eccentric woman who worked as a nanny and fought mental illness. Then there are interviews with the children she helped raise, all of whom are now adults, and what they say sneakily reveals more about themselves than the nanny they’re discussing. Directors John Maloof and Charlie Siskel weave this dense material with sensitivity, although there are times where a digression gets the best of them.