If it is a 1080p TV, then the TV always displays at 1080p. The source device and/or the TV will scale and frame convert the image to match the TV resolution and frame rate. When you power an HDMI device, it reads the TV's supported resolutions and audio formats over the HDMI cable, then it chooses whether or not to scale the image for the TV. The television then receives the signal and performs whatever image processing is needed to match the display needs. With many Source devices, you can use a setup menu to configure whether video should always be scaled up to 1080p. Some people prefer the image scaling of the television. However, if the Source does not upscale, it will limit the resolution of on-screen graphics, such as the program guide. So you'll probably prefer to scale your source devices to 1080p through their menus.
If this doesnt work; try this step by step
1) Turn on the Vizio TV.
2) Press the "Menu" button on the Vizio's remote to open the menu screen. 3) Press the arrow key to bring up the Wide menu. Press the down arrow to the "Wide" option and press "OK" to change the Vizio's picture settings to display 1,080-pixel signal with the best possible picture.