  UV, optical, and near-IR spectra of Vega have been combined to test our
understanding of stellar atmospheric opacities, and to examine the possibility
of constraining chemical abundances from low-resolution UV fluxes. We have
carried out a detailed analysis assuming Local Thermodynamic Equilibrium (LTE)
to identify the most important contributors to the UV continuous opacity: H,
H$^{-}$, C I, and Si II. Our analysis also assumes that Vega is spherically
symmetric and its atmosphere is well described with the plane parallel
approximation. Comparing observations and computed fluxes we have been able to
discriminate between two different flux scales that have been proposed, the
IUE-INES and the HST scales, favoring the latter. The effective temperature and
angular diameter derived from the analysis of observed optical and near-UV
spectra are in very good agreement with previous determinations based on
different techniques. The silicon abundance is poorly constrained by the UV
observations of the continuum and strong lines, but the situation is more
favorable for carbon and the abundances inferred from the UV continuum and
optical absorption lines are in good agreement. Some spectral intervals in the
UV spectrum of Vega that the calculations do not reproduce well are likely
affected by deviations from LTE, but we conclude that our understanding of UV
atmospheric opacities is fairly complete for early A-type stars.
