  JWST observations are leading to important new insights into exoplanetary
atmospheres through transmission spectroscopy. In order to harness the full
potential of the broad spectral range and high sensitivity of JWST, atmospheric
retrievals of exoplanets require a high level of robustness and accuracy in the
underlying models. We present the VIRA retrieval framework which implements a
range of modelling and inference capabilities motivated by early JWST
observations of exoplanet transmission spectra. This includes three
complementary approaches to modelling atmospheric composition, three
atmospheric aerosol models, including a physically-motivated Mie scattering
approach, and consideration of correlated noise. VIRA enables a cascading
retrieval architecture involving a sequence of retrievals with increasing
sophistication. We demonstrate VIRA using a JWST transmission spectrum of the
hot Saturn WASP-39 b in the $\sim$1-5 $\mu$m range. In addition to confirming
prior chemical inferences, we retrieve molecular abundances for H$_2$O, CO,
CO$_2$, SO$_2$ and H$_2$S, resulting in super-solar elemental abundances of
log(O/H)=$-2.0\pm0.2$, log(C/H)=$-2.1\pm0.2$ and log(S/H)=$-3.6\pm0.2$, along
with C/O and S/O ratios of $0.83^{+0.05}_{-0.07}$ and
$0.029^{+0.012}_{-0.009}$, respectively, in the free chemistry case. The
abundances correspond to $20.1^{+10.5}_{-8.1}\times$,
$28.2^{+16.3}_{-12.1}\times$ and $20.8^{+10.3}_{-7.5}\times$ solar values for
O/H, C/H and S/H, respectively, compared to C/H $= 8.67\pm0.35 \times$ solar
for Saturn. Our results demonstrate how JWST transmission spectroscopy combined
with retrieval frameworks like VIRA can measure multi-elemental abundances for
giant exoplanets and enable comparative characterisation with solar system
planets.
