The observation. In The Freedom of the Soul, Vivekananda's deciding question isn't "is this true?" — it's "does this make you strong?" "Are you strong? Do you feel strength?" He picks monism over dualism, freedom over bondage, the causeless soul over the bound one, because monism produces stronger human beings. He nearly admits it: he settles freedom-vs-bondage with "it is more consonant with all my feelings and aspirations" — a preference, not a proof. And he says outright he'd rather have "a little heart and no brain than be all brains and no heart."
The Live Wire (first framing). The whole majestic edifice — the deathless Self, the dethroned God, blame-no-one-and-rise — might be true, or might be the most strength-producing story a human can tell himself, chosen for that reason. Tell a person they are the causeless, free, deathless Self responsible for everything, and they become strong — not because it's true, but because believing it makes them so.
The Live Wire (second, sharper framing). This isn't a flaw in Vivekananda — it might be the most honest thing about him, and it generalizes. What if the function of the deepest metaphysical beliefs is never to be true but to be load-bearing — to make the person who holds them able to stand? Then the question "is monism true?" is a category error; the real question is "what kind of human does each belief build?" — and Vivekananda, forced to the wall, is betting that strong-and-maybe-wrong beats weak-and-maybe-right. Which means he's not a philosopher who happens to be a motivator; he's a belief-engineer who figured out that the truth-value of a worldview matters less than its tensile strength under load — and dressed that engineering as discovery because "I built this because it makes you strong" doesn't recruit, while "this is what you truly are" does.
Where it could go. Connects to The Freedom of the Soul and Strength Is the One Thing Needful. The behavioral-mechanics handshake is live: a belief selected for its effect on the believer is exactly what the influence corpus studies as ideology. Does the "self-fulfilling metaphysics" frame apply to the whole self-help tradition — and is the difference between a liberating self-fulfilling belief and a manipulative one only who chose it for you?