Let me start off that I know that Vegas can do a slide show. I just don't like it. So I purchased a slide show program which is "photo stage professional slide show producer" made by NCH.
I made the slide show there (Just pictures and animation zooms and pans). I then created a WMV file which I place into Vegas where needed.   After a Vegas render I get a consistent glitchs that are in the video. Every time I render the glitches are in different spots but on play back always in the same spots. In other words something is going on during the render. 
I take the slide show clip and put it in a new project all be itself and it is fine. 
One of the differences I see between the programs are the resolution specs.   Vegas is 1440x1080  with 23.976fps.    which is the default resolution for all my videos.  NCH is 1920x1080 at 29.97fps. 

I did a test by creating a new project. Using the default resolution of vegas I put in the slide show with its resolution. I figured it would glitch but it did not.  Unless there is some trick or solution I will have to go back to the vegas slide show feature. I don't like it but so far I have not found anything I like. I am using vegas pro 18 build 527. Windows 10.