I was aware of that sharpness is lost when i add the Stereoscopic 3D adjust FX with all parameters at zero.
Ok, that's some Vegas internal thing.

But today i discovered that when i import a full top-bottom 3D video, i lost a little details again, even when i don't add any FX to it!
Why is that?..

You can try that even with a still picture:

just search a 2D picture 1920x1080 with lot of fine details. Save as 2D.jpg. In Photoshop copy this picture below the first, so create a 1920x2160 "fake" 3D picture. Now import both to Vegas, set the 2nd one format to top/bottom (full) in the properties. Place these in two different track, and mute/unmute one. Examine your monitor closely, and you will see the sharpness lost on the 3D version.

Project properties 1920x1080, Left only, Quality: Best (Full) and Scale video to fit preview window: OFF

Sounds small thing, but to me it's a big problem, when i want to archive my 3D videos and maybe edit later. The file itself is sharp but sadly Vegas loose resolution when import it as top/bottom full (or sbs).

This is in Vegas11/12. Maybe another version can handle better?