Polymath versus Specialist for Vietnamese Teenagers 2026–2035: A Conditions-Strategy-Rule Synthesis of the Breadth-Depth Career Literature under AI-Era Repricing

A Vietnamese teenager curious about more than one field — typical of the academically-able 14–19 cohort — routinely receives a binary instruction: pick one and go deep, or stay broad and adaptable. Both ends are evidence-contestable as policy advice. This paper synthesises the empirical career-research, expertise, and AI-era labour-economics literature to replace the binary with a conditions-strategy-rule frame. The synthesis covers the deliberate-practice tradition (Ericsson 1993, Peak 2016, the Macnamara-Hambrick-Oswald 2014 meta-analysis bounding the claim), the Tetlock 2005 fox-versus-hedgehog finding and its 2015 Superforecasting operationalisation, the Epstein 2019 Range popularisation and its critiques, the career-capital and deep-work tradition (Newport 2012, 2016, 2024), the human-capital base (Becker 1964, Mincer 1974) with its breadth-of-skills extension (Lazear 2004, 2005), the boundaryless and protean career frameworks, the IDEO/IBM T-shape construct and its thin microempirical base, the Csíkszentmihályi 1996 creativity tradition with the Boden 1992 transformational-creativity decomposition, the AI-era exposure literature inherited from sibling I4.3, the motivation tradition (Pink 2009, Dweck 2017), and the Vietnamese career-trajectory and chuyên-track context. The Epstein Range thesis is presented honestly as a strong popular claim with thin empirical scaffolding: the conditional version (breadth dominates in wicked-domain forecasting and slow-feedback environments) is durable, the unconditional generalisation rejected. The synthesis distinguishes four constructs that popular discourse conflates: breadth as adolescent exposure, breadth as career-strategy, depth as deliberate practice in well-defined domains, and depth as risk under field-decline. The substantive contribution is a conditionsstrategy-rule table mapping domain-kindness, AI-substitution rate, market-thickness, and age-stage onto strategy choices, with operational rules for the Vietnamese 14–19 cohort. The proposed minimum-depth threshold — approximately three years of substantive effort in one chosen field by age 22 — operates as a non-negotiable floor on top of which breadth-as-strategy can be defended.