Don't buy a Vizio again or any bargain brand set if you want it to last. I'd look into some of the more reputable companies such as Toshiba, Sony, Samsung, etc even if you will be getting one of their low end models.
Are you looking for a LCD or plasma? You mentioned wanting a VGA so I'm assuming you will be looking this up to a PC? I'd go with LCD then. LED is coming down is price but I would stick with LCD to get the most bang for your buck.
The best advice I can give you is to use the filtering system on Wal-Mart's website to find one within your price and that meets your requirements. Then go onto CNET and Google professional reviews. Don't just go off the customer reviews as most people don't even know what they are talking about and don't have an eye for quality. I just did a quick search on Best Buy and there are several TVs under $500 that are worth looking into. Compare them all and decide what features and visual quality you are willing to sacrifice. It's a somewhat time consuming process so I can't really help you out too much especially since I don't know your exact criteria and what is most important to you in a TV.
Just curious, can you live without a HDTV for awhile? The best time of year to buy a TV is around Black Friday and the holidays. That would allow your money to go even farther and save up some extra green to throw at it as well. Right now, it looks like there are some decent sets for under $500 but nothing from a reputable company that offers 120HZ.
Wal-Mart really doesn't have that much. What about Best-Buy or Amazon?
The only sets I noticed on Wal-Mart's site within your price range that I would look into are:
http://www.walmart.com/ip/Toshiba-40-40FT2U/17201554http://www.walmart.com/ip/Philips-40-40PFL3706-F7/16874083http://www.walmart.com/ip/Philips-40-40PFL7705D-F7B/20565700The last one may really be worth looking into. Not only is it 120HZ but it is $100 under your budget. Unfortunately, it is refurbished. I'm not sure if I personally would buy a refurbished TV but it is certainly something for you to consider especially since it is going for almost $1,200 on Amazon.
http://www.amazon.com/Philips-40PFL7705D-F7-40-Inch-1080p/dp/B003ICXD0G Actually, the cheapest I see it for online is $734.80 from some random places online. It could potentially be the best bang for your buck.
You really need to read some professional reviews on these sets though. I've just provided you with a few sets that may be worth investigating.