  The X-shooter instrument on the VLT was used to obtain spectra of seven
moderate-redshift quasars simultaneously covering the spectral range 3000 Ang
to 2.5 microns. At z ~ 1.5, most of the prominent broad emission lines in the
ultraviolet to optical region are captured in their rest frame. We use this
unique dataset, which mitigates complications from source variability, to
intercompare the line profiles of C IV 1549, C III] 1909, Mg II 2800, and
Halpha and evaluate their implications for black hole mass estimation. We
confirm that Mg II and the Balmer lines share similar kinematics and that they
deliver mutually consistent black hole mass estimates with minimal internal
scatter (< 0.1 dex) using the latest virial mass estimators. Although no virial
mass formalism has yet been calibrated for C III], this line does not appear
promising for such an application because of the large spread of its velocity
width compared to lines of both higher and lower ionization; part of the
discrepancy may be due to the difficulty of deblending C III] from its
neighboring lines. The situation for C IV is complex and, because of the
limited statistics of our small sample, inconclusive. On the one hand, slightly
more than half of our sample (4/7) have C IV line widths that correlate
reasonably well with Halpha line widths, and their respective black hole mass
estimates agree to within ~0.15 dex. The rest, on the other hand, exhibit
exceptionally broad C IV profiles that overestimate virial masses by factors of
2-5 compared to Halpha. As C IV is widely used to study black hole demographics
at high redshifts, we urgently need to revisit our analysis with a larger
sample.
