This is due to the aspect ratio you chose (maybe without meaning to) for the image. It's normally set to standard PC 4:3 which fits, as the name implies, on the screen (assuming you don't have some odd monitor size). If I choose standard PC vertical 3:4 I get the same bars as you. Vue is showing you the non-renderable areas due to the chosen aspect ratio.

Just set it back to standard in the render settings and it'll be OK.

Steve