I’ve had this problem when I tried to do a smart render and didn’t realize that my media resolution/aspect ratio was different from my project properties, but it never came up with an mp4 render. First thing I would do is check the resolution and pixel aspect ratio of your media, then your project properties, and make sure they’re all the same. Would you post those, and what your render settings are?
Of course, I’m half tempted to say that Vegas is just buggy.
http://www.scenethroughglass.com