  In order to protect society from space weather impacts, we must monitor space
weather and obtain early warnings for extreme events if possible. For this
purpose, the European Space Agency is currently preparing to launch the Vigil
mission towards the end of this decade as a space-weather monitor at the fifth
Lagrange point of the Sun--Earth system. Vigil will carry, amongst other
instruments, the Plasma Analyzer (PLA) to provide quasi-continuous measurements
of solar wind ions. We model the performance of the PLA instrument, considering
typical solar wind plasma conditions, to compare the expected observations of
PLA with the assumed input conditions of the solar wind. We evaluate the
instrument performance under realistic, non-equilibrium plasma conditions,
accounting for temperature anisotropies, proton beams, and the contributions
from drifting $\alpha$-particles. We examine the accuracy of the instrument's
performance over a range of input solar wind moments. We identify sources of
potential errors due to non-equilibrium plasma conditions and link these to
instrument characteristics such as its angular and energy resolution and its
field of view. We demonstrate the limitations of the instrument and potential
improvements such as applying ground-based fitting techniques to obtain more
accurate measurements of the solar wind even under non-equilibrium plasma
conditions. The use of ground processing of plasma moments instead of on-board
processing is crucial for the extraction of reliable measurements.
