Insignia LCD-based sets used to use LG parts and IPS panels years ago, but that's (likely) no longer the case...they can come from various suppliers, notable among them Hon Hai Precision (Foxconn) and XOCECO (Prima) depending on the model. They use VA based panels but I'm not sure from which supplier. Insignia products come from many suppliers...when they had plasmas, they were rebadged Samsungs, I've seen their AV receivers made by both Onkyo and Inkel, DVD players from Funai and Desay. Generally good quality stuff without lots of features. Their TVs look pretty decent.
Vizio sets generally have more features and slightly better image quality. E.g. pseudo-local dimming, wi-fi, passive 3D. I personally don't find their picture quality to be that amazing, rather pretty "solid." I've experienced numerous problems with poorly implemented remote sensors on multiple M-Series TVs that call to attention that Vizio may be over-reaching in their attempt to be seen as a first-tier manufacturer. There were also issues with initial runs of E-series sets that have been fixed. afaik
all Vizios are manufactured by Amtran who also makes JVC TVs. You get a lot of bang for the buck with Viz but their reliability and warranty service doesn't seem to be on par with the majors, HDGURU even did a piece on this.
When it comes to which I'd personally buy, probably neither, Id take a look at the new 50" Sony 450A which goes for $600 -- if it's anything like the 40" version, it'll offer very good image quality. If you're open to a plasma, then a 50" Panasonic S60 would offer better image quality than either option with better reliability.