VirtualDub & 3D movies taken with a single miror example of Screen Vision use (for export to StereoMovieMaker) :

First frame of 3D-movie in VirtualDub

We have to select the right portion, miror it, save it. Then get the equivalent portion on the left side : same zone, same size (otherwise SMM will report an error) and save it too.

Lets crop the mirored side ...

For compatibility, lets check that both sides are multiple of 4 ...
If not go back one step.

This is the crop...

Select the horizontal flip filter and apply it ...

Open Screen Vision, give it focus with the mouse and set the windows size to the same values (here 372*384)

Now still with focus on Screen vision, move the mouse to the center of the output movie picture (about where I put a black dot),
to fill the Screen Vision window with the picture...

See below that we can keep the trace of the image and put it were we want ...

Now it is time to save the first right part of the stereo movie, in uncompressed format (SMM does not like compressed files !)

Then we go back to cropping the left part of the 3D movie !

See how I use a recognizable feature (here top of roof) to align the windows...

Now we do the cropping as before..and thanks to Screen Vision it is easy to see where to crop !

Don't forget to verify that the crop movie has the same size as the other one !

Cropping done... You can already have an idea of the result :

here 3D-// vision

here cross-eyes vision

You just have to release and close Screen Vision (Hit space when in focus) and
to save the left part of the movie.

After just open the 2 movies made in SMM and ... !