  The direct observation of cold and temperate planets within 1 to 10 AU would
be extremely valuable for uncovering their atmospheric compositions but remains
a formidable challenge with current astronomical methods. Ground-based optical
interferometry, capable of high angular-resolution imaging, offers a promising
avenue for studying these exoplanets, complementing space-based observations.
Our objective is to explore the fundamental limits of dual-field interferometry
and assess its potential for characterizing exoplanets in reflected light using
the Very Large Telescope Interferometer (VLTI). We developed analytical
expressions to describe the performance of dual-field interferometry and
integrated these with simulations of atmospheric wavefronts corrected by
extreme Adaptive Optics. An analytical solution for optimal phase apodization
was formulated to enhance starlight rejection when injected into a single-mode
fibre. This framework was applied to determine the detectability of known
exoplanets in reflected light across various wavelength bands for both the
current VLTI and a proposed extended version. Our results indicate that
employing shorter wavelengths improves detectability, enabling at least seven
Jupiter-mass exoplanets to be observed in the J band with current VLTI's
baselines. Adding new baselines with lengths beyond 200 meters significantly
enhances VLTI's capabilities, increasing the number of detectable exoplanets
and revealing potential habitable zone candidates such as $\tau$ Ceti e and
Proxima Centauri b. To substantially improve the VLTI's exoplanet
characterization capabilities, we recommend developing instrumentation at
wavelengths shorter than 1$\,\mu$m, as well as the addition of a fifth Unit
Telescope (UT5).
