  We investigate the highly frustrated spin and orbital superexchange
interactions in cubic vanadates. The fluctuations of $t_{2g}$ orbitals trigger
a {\it novel mechanism of ferromagnetic interactions} between spins S=1 of
V$^{3+}$ ions along one of the cubic directions which operates already in the
absence of Hund's rule exchange $J_H$, and leads to the C-type
antiferromagnetic phase in LaVO$_3$. The Jahn-Teller effect can stabilize the
orbital ordering and the G-type antiferromagnetic phase at low temperatures,
but large entropy due to orbital fluctuations favors again the C-phase at
higher temperatures, as observed in YVO$_3$.
