This is a common complaint. To be frank I don't have the exact term for your Vizio, but most recent LCD and plasma TVs have the ability to "zoom" or adjust "picture format" (those are two commonly used terms) depending on the signal source and format.
it has to do with the fact that currently there are both classic style format 4:3 signal sources as well as new HD 16:9 signal sources and on top of that it may happen that a 4:3 signal source is distributing adjusted image from a 16:9 or other signal source. Think cinema adjusted to run on classic TV - remember the vertical strip of image with two huge black areas below and above it?
Well the ZOOM or PICTURE FORMAT allows to adjust such an image so that you cut the "void" are by scalling (enlarging) the visible area to fit the whole screen.
A problem then appears when the signal source is normal and showing full screen. Then, because of the enlargement, you start missing some of the picture. Before there was nothing there because it was showing adjusted wide (cinema) format to the TV box format, but when the new TV box format gets enlarged it sure starts missing around the edges.
That's what you're seeing.
More and more TVs do not have the ability to adjust that through the TV itself. Don't ask me why, but they can only do it through the remote.
So if you have an original remote there MUST be a button there that's called ZOOM or FORMAT.
It does the trick.
Hope this helps. If it does do not forget to rate it.