  We analytically derive an equation describing vesicle evolution in a fluid
where some stationary flow is excited regarding that the vesicle shape is close
to a sphere. A character of the evolution is governed by two dimensionless
parameters, $S$ and $\Lambda$, depending on the vesicle excess area, viscosity
contrast, membrane viscosity, strength of the flow, bending module, and ratio
of the elongation and rotation components of the flow. We establish the ``phase
diagram'' of the system on the $S-\Lambda$ plane: we find curves corresponding
to the tank-treading to tumbling transition (described by the saddle-node
bifurcation) and to the tank-treading to trembling transition (described by the
Hopf bifurcation).
