  Metal hydrides and oxides are important species in hot-Jupiters since they
can affect their energy budgets and the thermal structure of their atmospheres.
One such species is VO, which is prominent in stellar M-dwarf spectra. Evidence
for VO has been found in the low-resolution transmission spectrum of WASP-121b,
but this has not been confirmed at high resolution. It has been suggested that
this is due to inaccuracies in its line list. In this paper, we quantitatively
evaluate the VO line list and assess whether inaccuracies are responsible for
the non-detections at high resolution in WASP-121b. Furthermore, we investigate
whether the detectability can be improved by selecting only those lines
associated with the most accurate quantum transitions. A cross-correlation
analysis was applied to archival HARPS and CARMENES spectra of several M
dwarfs. VO cross-correlation signals from the spectra were compared with those
in which synthetic VO models were injected, providing an estimate of the ratio
between the potential strength (in case of a perfect model) and the observed
strength of the signal. This was repeated for the reduced model covering the
most accurate quantum transitions. The findings were fed into injection and
recovery tests of VO in a UVES transmission spectrum of WASP-121b. We find that
inaccuracies cause VO cross-correlation signals in M-dwarf spectra to be
suppressed by about a factor 2.1 and 1.1 for the complete and reduced line
lists, respectively, corresponding to a reduced observing efficiency of a
factor 4.3 and 1.2. The reduced line list outperforms the complete line list in
recovering the actual VO signal in the M-dwarf spectra by about a factor of
1.8. Neither line list results in a VO detection in WASP-121b. Injection tests
show that with the reduced efficiency of the line lists, the potential signal
as seen at low resolution is not detectable in these data.
