Opaque overlay becomes transparent when fading out both video tracks in Sony Vegas The original problem I had was to put an overlay (static image) on a video track. All guides/tutorials I found does this by putting the image into a separate video track. I would prefer attaching the image to the actually video clip instead, but fair enough, it works.

However, when I try to fade out the video + overlay, they do this at different "pace". The video is recorded from gameplay, and I want to cover the chatbox with a black image. It works fine, and I can't see through the image at all. But when I fade out the video track and the image, I can pause in the fading and actually see the chat box. This is extremely frustrating, and not at all what I want of course.

I have tried different things, fading out by dragging the right corner, and a composite level envelope, but it doesn't help. It doesn't matter if it transitions into another video clip or to black, same problem occurs anyway.

Here is an example of how it looks:
This is the expected behavior for fading two video tracks.  Video tracks are blended in order to build up what you see as an end result.  When you make the image mask (the overlay) for the chat pain start fading to transparent, it allows the images underneath to be seen.  So if it is at 70% transparent, it lets through 30% of the underlying layers, which in turn is letting through 30% of the layer under it.

To get around this, you need to fade the combined video rather than the individual videos.  The easiest way to do this in your case is going to be to use parent/child compositing relationships.  By telling Vegas that the two clips are related and should have effects applied to both, it should allow you to have an effect applied to the sum total of both tracks instead of needing to apply it to both individually.

Sony actually already has a pretty nice write up on the feature available here.