Laboratory searches determine the mass and kinetic mixing of dark-photon dark matter. Cosmic microwave background isocurvature probes how its abundance depends on inflationary initial conditions. We formulate this dependence through the logarithmic response $q_{\mathrm{eff}}=\partial \ln n_{A^{\prime }}/\partial \ln r_i$ of the final vector number to the primordial dark-Higgs displacement. For a smooth local abundance map the separate-universe relation connects $q_{\mathrm{eff}}$ to cold dark matter isocurvature. The uncorrelated Planck limit applies when the dark Higgs is a light and energetically subdominant spectator with weak inflaton mixing. In the quadratic conserved-number branch displacement-independent transfer gives $q_{\mathrm{eff}}=2$. The all-dark-matter bound then requires $r_i/H_{*}>3.5\times {10}^4$. We calculate the dynamical map by evolving the radial mode and its tangent response through a smooth Abelian-Higgs crossover. The post-transition orbit gives the comoving action. The map contains positive, negative, and near-zero responses at finite transferred action. Tanh and error-function crossovers retain these branches. A thermal-mass profile and finite inflationary quartics shift their locations continuously. Stochastic duration and radiative stability then determine realization regions. Early symmetry breaking and momentum redshift add the remaining conditions. Low-energy measurements and the CMB response can distinguish Higgsed-vector cosmologies with identical present-day $(m_{A^{\prime }},\epsilon )$ and relic abundance.

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• Cosmology
• Dark matter
• Inflation