I cannot answer all your questions; I am very much an amateur who has done some reading and has a little experience.
If you watch a video with poor video quality but good sound it generally seems acceptable, but you will really notice bad sound with good video. At least the dialogue should be easily intelligible, and background noises not distracting. This can usually be done with inexpensive equipment. (I have one very old Radio Shack mic that sounds nice, but I tried some new, cheap name-brand mics that were really bad, and I returned them after just a few seconds of testing. I gradually, reluctantly, gave in to the idea that I needed better equipment, and needed to spend more money. However, trying to economize, I bought a few things that turned out still to be not very satisfactory. So I now know some rather expensive stuff that is fine (I bought some good stuff on eBay, but you need time and patience to get great deals), but I don't know where to tell anyone to draw a line. Pristine sound may sometimes be required for broadcast, but pursuit of such sound may be unnecessary - you should ask some broadcasters what they expect as to both audio and video. Try to get some experience with someone producing stuff you like.
I cannot compare different cameras, but some of the Vixias do produce nice video; discussion groups are full of people using them commercially for events, like weddings. I don't know when they would be, or would not be, acceptable for broadcast. I have a friend who does lots of documentary work; he doesn't care for HD video (not quite yet widely adopted, and he doesn't like the lossy compression of the signal), but he swears by 3CCD cameras, which is probably the dividing line between consumer and pro or prosumer cameras. However if you use a consumer camera, those audio adapters I mentioned are more expensive than you might think, and they're a bit of an inconvenience; also the advantages of the consumer cameras' small size start to be lost. And once you go there, all those good microphones will tug at your purse strings.