  Vector bosons heavier than $10^{-22}$ eV can be viable dark matter candidates
with distinctive experimental signatures. Ultralight dark matter generally
requires a non-thermal origin to achieve the observed density, while still
behaving like a pressureless fluid at late times. We show that such a
production mechanism naturally occurs for vectors whose mass originates from a
dark Higgs. If the dark Higgs has a large field value after inflation, the
energy in the Higgs field can be efficiently transferred to vectors through
parametric resonance. Computing the resulting abundance and spectra requires
careful treatment of the transverse and longitudinal components, whose dynamics
are governed by distinct differential equations. We study these equations in
detail and find that the mass of the vector may be as low as $10 ^{ - 18 }$ eV,
while making up the dominant dark matter abundance. This opens up a wide mass
range of vector dark matter as cosmologically viable, further motivating their
experimental search.
