Are you somehow importing .VEG project files into DVD Architect, and having it render those to MPEG-2 directly? (I’m not sure if that’s even possible!)
If this is the case though, it sounds like there’s an Aspect Ratio setting gone awry somewhere. I’ve found that with Vegas, slight differences can creep in even when the Ratio settings are the same, when you begin switching around programs.
My suggestion, if you aren’t already doing this: open Vegas and use File > Render As to save an .AVI file of the video. Use an aspect ratio setting which matches the original project (1.45 or whatever – copy whatever is in your Project Properties screen into the Aspect Ratio dialogue in the “Export” window). Then use a lossless codec like Sony YUV Codec, and save the AVI file to disk. Then, import that into DVD Architect.
If you still get the same problem, make sure that the lens flare isn’t out of place in the source AVI file you just rendered. If it is, go back to Vegas, find the element with the lens flare on it, and make sure its Aspect Ratio is set correctly.