While I'm not familiar with this particular set the resolution should be dependent on the source not the TV.
If you are feeding it a Standard DVD through the Component inputs then 480 is what you should get as that is the DVD standard, a Bluray disc through the HDMI will output 1080p, as far as I know there are no settings on the set itself that will change this.
The manual for this set is available here:
http://www.vizio.com/e320vl.html#support