Set your refresh rate for 60hz. That's the refresh rate these panels use. Using a refresh rate of 75hz won't help and could be responsible for your problems if the panel isn't able to handle the "conversion" well. Also, what is the native resolution of your display? The 42" Vizios that I know of run at either 1366x768 or 1920x1080. 1280x768 is either going to end up being stretched or displayed with pilars. Also, make sure your display is not set to stretch/fill/overscan/zoom mode. If it's not doing a straight 1:1 display of the image, the image will look like crap.
Get your settings straightened out and see if it makes any difference.
60hz
1366x768 or 1920x1080
1:1 pixel mapping
Edit:
Oh. I see you said plasma. I assumed LCD. That narrows down the possibilities. The single 42" plasma listed on vizio's site has a native resolution of 1024x768 which means the image is going to look squished if you use a 1:1 pixel mapping. What a weird choice on their part. Well, it is what it is. I have no idea what the native refresh rate is on this display, tho I suspect it's the same 60hz as LCDs since they're based around the 60hz 720p HDTV spec.