  Here we provide uniqueness of vanishing viscosity solutions to sub-Riemannian
mean curvature flow problem, which was known only far from characteristic
points or under special symmetry condition. We employ vanishing viscosity
approach and look for solutions as limit of solutions to approximating flow,
which is well defined also at characteristic points, and estimate the rate of
convergence of the approximating solutions. The results are provided in the
settings of both 3-dimensional rototranslation group $\text{SE}(2)$ and
Heisenberg group $\text{H}^1$ and they are particularly important due to their
relation to surface completion problem of model of the visual cortex.
