That's normal behaviour and to my knowledge cannot be altered. I suspect it's also not showing every frame. Possibly it's done to reduce GPU load. You will also note that during rendering the project settings temporarily alter to your render settings.
Please post images direct to this forum using the upload button rather than to an external site.
Cool, thanks :D Yes, I finished rending regardless, and it looks like the same quality. Now I'm uploading it on Youtube (as a private test video, still not done editing it) and it's taking the same amount of time to upload, which means it's still the same HD quality :D
Oh, and thanks for showing where to upload the image here :D
I was so frustrated before trying to figure out what was going on with the preview screen, that I overlooked that "upload image here" button :)
In the latest Vegas versions the shrinking of the previewscreen during rendering is also dependant on the rendertemplate compared to the original sourcefile and the projectsettings, look how this goes in VPro 18: