  Photometric and astrometric monitoring of directly imaged exoplanets will
deliver unique insights into their rotational periods, the distribution of
cloud structures, weather, and orbital parameters. As the host star is occulted
by the coronagraph, a speckle grid (SG) is introduced to serve as astrometric
and photometric reference. Speckle grids are implemented as diffractive
pupil-plane optics that generate artificial speckles at known location and
brightness. Their performance is limited by the underlying speckle halo caused
by evolving uncorrected wavefront errors. The speckle halo will interfere with
the coherent SGs, affecting their photometric and astrometric precision. Our
aim is to show that by imposing opposite amplitude or phase modulation on the
opposite polarization states, a SG can be instantaneously incoherent with the
underlying halo, greatly increasing the precision. We refer to these as vector
speckle grids (VSGs). We derive analytically the mechanism by which the
incoherency arises and explore the performance gain in idealised simulations
under various atmospheric conditions. We show that the VSG is completely
incoherent for unpolarized light and that the fundamental limiting factor is
the cross-talk between the speckles in the grid. In simulation, we find that
for short-exposure images the VSG reaches a $\sim$0.3-0.8\% photometric error
and $\sim$$3-10\cdot10^{-3}$ $\lambda/D$ astrometric error, which is a
performance increase of a factor $\sim$20 and $\sim$5, respectively.
Furthermore, we outline how VSGs could be implemented using liquid-crystal
technology to impose the geometric phase on the circular polarization states.
The VSG is a promising new method for generating a photometric and astrometric
reference SG that has a greatly increased astrometric and photometric
precision.
