While I cannot answer your question directly, I would say that personally, I would take customer/tech support availability into mind when looking at the costs.
I bought a westinghouse 37" 1080P a few years ago and while I haven't had problems with it, many people have not had the best experience with customer support and getting TV issues resolved through westinghouse in or out of warranty. On the flip side, samsung users have reported getting out of warranty replacement/repairs done without much hassle.
At the very least, I would recommend you check out AVS forums and browse through the LCDTV section to see if anyone has any serious negative comments about either TV and whether Vizio stands behind their customers as strongly as Samsung, LG, Pioneer, etc.
Different people value "peace of mind" differently depending on their planned use of the TV or even their personality.
As for is it worth the extra $300, do they have equal feature offerings, equal picture quality, equal cosmetic appeal, input/outputs, warranty length, ...?