I have that issue on occasion, and all I do is trim off 1 frame from the end of the clip that's causing the problem.

That's basically the problem - you have a blended frame. Something else to consider with Vegas is that certain effects can appear or disappear depending on the level of detail you are using to watch the timeline (Draft/Preview/Good/Best). For example, when I was making soccer videos for my kids, I would sometimes render out fast action segments in Preview quality, because it reduced the instances of field artifacts. I put these segments back into the timeline, and then rendered out the whole video, including those segments, in Good quality (which is usually fine for most projects.)