I can think of a few things off the top of my head:
1) Samsung charges more because they can. Only half kidding.
2) Vizio started out targeting the lower half of the mass-market, at which they have been very successful. That's less true today, as they're selling better, fuller-featured sets, but they still aren't as full-featured by comparison with tier-one brands. For instance, AFAIK, you won't find full CMS settings on any Vizio, as the settings options are limited.
3) Processing. Specifically picture processing, and this is something quite apart from the panel's characteristics. I'd be willing to bet that Vizio is at least a year behind Samsung in that regard.
That last point requires some elaboration, so I hope you'll indulge me.
Plasma fans to the contrary, LED/LCD has seen significant, incremental improvements over the last 6-7 years. I got one of the first 120 Hz LCDs with frame interpolation in 2007, I think it was. (A Philips 47", when they still sold good TVs in North America.) At that time and for a few years thereafter, the difference between top-end and bottom-end sets as to picture processing was very obvious (to me and many others, anyway). Over those years, motion has improved, interpolation (if you care for it) has improved, you can now have backlight scanning, etc. Now, these improvements eventually find their way into lower-end sets, of course. But tier-one manufacturers, of which Samsung is unquestionably one, are always improving their sets. Their reputation depends on it.
How much difference, then, is there between those two TVs as to processing? I confess I dunno, but probably less than in years past. Evidently not enough for the average TV buyer to care much about. For AVS members, it's more arguable.
Mind you, I hold no brief for either brand. I still remember the Samsung capacitor fiasco and the terrible way they handled it. So I've avoided them. I considered Vizio when shopping for a 70" set, but decided on a Sharp instead, in part at least for the fine control possible in the picture settings. I haven't regretted it.
Good luck whatever you get.Edited by fritzi93 - 10/8/13 at 6:09pm