  Available form factor parametrizations for $B\rightarrow D^*l\nu$ imply
different theoretical assumptions and different treatments of theoretical
uncertainties. They give results for $\vert V_{cb}\vert$ whose central values
are apart by up to $8\%$. The way the Caprini Lellouch Neubert (CLN)
parametrization has been used in experimental analyses sets theoretical
uncertainties of the Heavy Quark Effective Theory (HQET) results on slope and
curvature of the form factor ratios $R_1$ and $R_2$ to zero. Furthermore, the
relation of curvature and slope of the axial form factor $A_1$ is fixed to the
HQET central value. In view of the current experimental precision these
uncertainties cannot be neglected any more. Using the Boyd Grinstein Lebed
(BGL) parametrization and taking into account theoretical uncertainties in a
conservative way, we extract $\vert V_{cb}\vert$ from recent preliminary Belle
data and the world average of the total branching ratio. We include an
$\mathcal{O}(10\%-20\%)$ theoretical uncertainty of HQET input due to unknown
corrections beyond NLO which were neglected in all previous analyses. This is
important for reliable extractions of $\vert V_{cb}\vert$ as well as precision
tests of the Standard Model with robust predictions of the lepton flavor
nonuniversality observable $R(D^*)$ and the $\tau$ polarization asymmetry
$P_{\tau}$. Including input from Light Cone Sum Rules (LCSRs) we find $\vert
V_{cb}\vert = 40.6\left(^{+1.2}_{-1.3}\right)\cdot 10^{-3}$, $R(D^*) =
0.260(8)$ and $P_{\tau}=-0.47(4)$. Without LCSRs we find $\vert V_{cb}\vert =
41.5(1.3)\cdot 10^{-3}$ and the same results for $R(D^*)$ and $P_{\tau}$. The
$R(D^*)$ anomaly is persistent, but its statistical significance is slightly
reduced to 2.6$\sigma$.
