If you go to the Nvidia home page and look at the stereoscopic 3d page you will see under the specifications tab that it is only supported by a dvi connection. That means that the dvi to vga connector changes your input to vga and therefore not supported.
thnx inzone but this is not a solution of my problem
in present i cant buy a new monitor
because its out of my budget.....
m searching for a software or driver which can solve my problem.......
and stereoscopic 3d also works on vga connection
i was using stereoscopic 3d on my old card GT8400