  We introduce a microscopic model which unravels the physical mechanisms
responsible for the observed phase diagram of the $R$VO$_3$ perovskites. It
reveals a nontrivial interplay between superexchange, the orbital-lattice
coupling due to the GdFeO$_3$-like rotations of the VO$_6$ octahedra, and
orthorhombic lattice distortions. We find that the lattice strain affects the
onset of the magnetic and orbital order by partial suppression of orbital
fluctuations. The present approach provides also a natural explanation of the
observed reduction of magnon energies from LaVO$_3$ to YVO$_3$.
