If you route audio through the T.V, then your Aunt doesn't have to worry about changing inputs on the Vizio to get sound with her picture if she goes from cable to blu-ray. The only issue is if the Vizio sound bar wants to do something fancy with a 5.1 sound signal, since most T.Vs only output audio in stereo (digital and analog).
My sound bar is 2.1, and I have everything going through my T.V because that's fewer cables, and fewer inputs to change when going between devices (although, I only have 2 devices hooked up). Also, if you have two speakers, you might as well have a less compressed audio signal rather than a more compressed 5.1 audio signal that your speakers then have to downmix to send through two speakers.
As for analog (RCA) vs digital, I think audio is noticeably better with digital on my sound bar (less hissing, less distortion). However, that could be due to any number of things, and you could very well not even notice a difference between the two.