  We develop a vortex metal theory for partial filled Landau Level at
$\nu=\frac{1}{2n}$, whose ground state contains a composite Fermi surface(FS)
formed by the vortex of electrons. In the projected Landau Level limit, the
composite Fermi surface contains $\frac{-\pi}{n}$ Berry phase. Such fractional
Berry phase is a consequence of LL projection which produces the GMP guiding
center algebra and embellishes an anomalous velocity to the equation of motion
for the vortex metal. Further, we investigate a particle-hole symmetric bilayer
system with $\nu_1=\frac{1}{2n}$ and $\nu_2=1-\frac{1}{2n}$ at each layer, and
demonstrate that the $\frac{-\pi}{n}$ Berry phase on the composite Fermi
surface leads to the suppression of $2k_f$ back-scattering between the PH
partner bilayer, which could be a smoking gun to detect the fractional Berry
phase. We also mention various instabilities and competing orders in such
bilayer system including a $Z_{4n}$ topological order phase driven by quantum
criticality.
