  NMR and magnetization measurements in Li$_2$VOSiO$_4$ and Li$_2$VOGeO$_4$ are
reported. The analysis of the susceptibility shows that both compounds are
two-dimensional $S=1/2$ Heisenberg antiferromagnets on a square lattice with a
sizeable frustration induced by the competition between the superexchange
couplings $J_1$ along the sides of the square and $J_2$ along the diagonal.
Li$_2$VOSiO$_4$ undergoes a low-temperature phase transition to a collinear
order, as theoretically predicted for $J_2/J_1 > 0.5$. Just above the magnetic
transition the degeneracy between the two collinear ground states is lifted by
the onset of a structural distortion.
