Hello!
This is something that started happening like few weeks ago and I never experienced it in 10 years of using Vegas. I'm currently on the latest version.
I always export my videos in HD quality, and sometime I use 4k footage mixed with HD one.
The problem that I have now is, when I render a video, all clips that are in 4k are not downscaled to fit the screen as they normally are in a preview window, but instead only top left part of a clip is visible.
I didn't change any options, at least not intentionally and have no idea why it's happening now, someone please help!
Preview window:
Rendered video: