As someone who has owned a Philips and a Vizio, I have to say go with the Philips. I had a Vizio for 6 months and was happy with it, but the HDMI ports broke and I had to return it to Costco. The floor manager said he had seen quite a few come back for this exact reason, but rarely gets the Philips back due to malfunction. At the time of purchase, both TV's were within 50 dollars of each other, so I cant imagine he was pushing the Philips to up my spending.
The models in question I owned:
Vizio GV47LF and Philips 47PFL7422D/37
For HD quality they are quite comparable. The Vizio tended to produce whiter whites (unrealistic) with the Philips producing better overall accuracy, but not quite as vibrant.
However for SD the Philips wins hands down. The scalar in it is simply better than the Vizio's. When I had the Vizio, watching SD was ok, but always a bit of a problem, and sometimes Id get complaints on its quality. With the Philips I don't get any complaints from anyone anymore.
Also the Philips worked better when attached to my Media Center, as I had no problems getting 1920x1080 over HDMI, which the Vizio was a little sketchy with. However most of these problems have been addressed from what I hear. Still if you have a problem, theres no easy firmware upgrades with Vizio. Philips has a USB port, and upgrading firmware is about a 3 minute process.
That being said, the Vizio is still a good TV, and if most of your content is going to be HD (Blu-Ray, Xbox 360/PS3, HD-Cable/SAT etc.) then go with the Vizio to save some money. But if you want better SD viewing, the Philips is worth it. Also the firmware upgrade via USB is a great bonus.