IF the Vegas preview window, with the computer to studio effect OFF looks the same as your Sony AVC render with the computer to studio effect ON when played in a video player then…
Your source video is full range.
Sony AVC encodes expects studio range levels.
The Vegas preview window only shows correct levels if it is given full range data.
Therefore for your playback to look the same as your video encode you have to do different things to your video footage to get same look for editing and video player playback.
There are many ways to go about this and I’ll suggest one.
Make sure all your source footage is studio levels on the timeline.
If source footage is full range then put a media fx, computer to studio effect on each media file. If your source is already studio, then there is nothing to do.
On your output fx (in the preview window), put a studio to computer effect.
During editing, the levels output effect should be enabled. When you encode (render as), this levels adjustment should be disabled.
Now your preview window and final output in a media player window should match.
If you have more contrast than you like then add a media/track/event effect to reduce your contrast, or whatever you do not like about saturation, brightness, gamma etcetera.
Hope this helps and that I was clear enough.
This whole mess comes about because many new AVC cameras output full range data.
The Vegas preview window does not have a levels adjustment option like the external preview device has. A Sony mistake IMO.