  Vortex lattice (VL) elastic response is analyzed within nonlocal London
model. The squash modulus turns zero at the field $H_{\Box}$ where VL undergoes
square-to-rhombus structural transition. In the field region $H>H_{\Box}$ where
the square VL is stable, the rotation modulus turns zero at a field $H_r$
indicating instability of the square VL with respect to rotations. The shear
modulus depends on the shear direction; the dependence is strong in the
vicinity of $H_{\Box}$ where the square VL is soft with respect to the shear
along [110]. The $H$ dependences of elastic moduli are evaluated for
LuNi$_2$B$_2$C.
