  We revisit inflation coupled with vector fields employing kinetic coupling in
the comoving gauge. It is known that there is a cumulative effect
$\tilde{I}N^2$ on curvature power spectrum. For a large number of e-foldings
$N$, this contribution is so significant that it could violate observational
constraints when the ratio of kinetic energy between vector fields and inflaton
$\tilde{I}$ is not extremely small. In this paper, we explore the regime where
$\tilde{I}\gg 1$, a realm that has not been extensively explored due to the
limitations of perturbative methods. We found that the entropy perturbation
becomes heavy in this regime and the cumulative effect decays away on
super-horizon scales. Consequently, the power spectrum retains its scale
invariance in the decoupling limit. By straightforwardly integrating out the
heavy modes near horizon-crossing, we derive a low-energy effective field
theory describing a massless adiabatic perturbation with an imaginary speed of
sound $c_s^2= -1/3$. Namely, the inflation with vector fields presents a
potential mechanism for generating primordial black holes.
