  CK Vul or Nova 1670 is an enigmatic eruptive object which underwent a
stellar-merger event recorded by seventeenth-century observers. Its remnant was
recently recovered at submillimeter wavelengths, revealing gas of an
extraordinary isotopic composition indicative of past processing in the CNO
cycles and partial He burning. Here, we present long-slit optical spectra of
the remnant acquired with X-shooter at the Very Large Telescope at an
unprecedented sensitivity and spectral coverage. The spectra cover features of
key elements - including H, He, C, N, and O - at ionization degrees I-III. A
classical analysis of the spectra allows us to identify several
spatio-kinematic components in the probed part of the nebula at electron
temperatures of 10-15 kK and densities of 200-600 cm$^{-3}$. We find that the
nebula is most likely excited by shocks rather than by direct radiation of the
stellar remnant. We provide a detailed analysis of the elemental abundances in
the remnant and find that helium is twice more abundant than in the Sun.
Nitrogen is also overabundant with a N/O ratio ten times larger than the solar
one. These anomalous abundances strongly indicate that the observed gas was
processed in CNO cycles of H burning, consistent with the submillimeter
studies. Additionally, sub-solar abundances of heavier elements, such as Ne, S,
and Ar, suggest that the progenitor of CK Vul was formed from material poorer
in metals than the Sun and was therefore an old stellar system before the 1670
eruption.
