A contentious family grappling with a matriarch’s dementia over the Christmas holiday might not seem an obvious template for humor, but in “Dot,” playwright Colman Domingo sees the absurdity and the human comedy in the messy, volatile, all-too-real family dynamic. Though the show, premiering at Off Broadway’s Vineyard Theater, occasionally veers to the maudlin, there’s much authentic emotion and comfort in this loss-of-memory tale. Problem is, the playwrights stuffs his overlong play with enough plotting, themes and mid-life crises for multiple works, and the usually savvy helmer Susan Stroman mis-dials the acting to an unvarying degree.

In this companion piece to Domingo’s autobiographical “A Boy and His Soul” and “Wild With Happy,” something is a little off about Dotty, the wide-eyed, feisty matriarch of a black, middle-class West Philadelphia family. At first it appears her adult daughter, lawyer Shelly (Sharon Washington), is over-reacting to anything her mother says or does, but it soon becomes clear that there’s a good reason for her stressed-out behavior: Though at times lucid and wickedly funny — she compares her daughter’s new hairstyle to “a mean pineapple” – Dotty shows increasing signs of Alzheimer’s.

Adding stress are Shelly’s siblings, who are too self-involved in their own lives to accept what is happening and help their sister share the burden of dealing with their quickly-declining, widowed mother.

Shelly is at a near-hysterical level even before once-golden-boy brother Donnie (Stephen Conrad Moore, solid) and husband Adam (Colin Hanlon, the likable in-law) arrive for a holiday visit from New York City. It’s hard to focus on mom when the men are having marital issues of intimacy and choice.

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