Television, ironically, has never been very good at the vision thing. As for its prefix, well, "tele" is Greek for "at a distance," and that seems to be exactly what Madison Avenue has been doing in recent months: distancing itself from television. The word, if not the medium. MediaVest, the giant media shop that buys for Coca-Cola, Kraft, Procter & Gamble, and others, has dropped the word altogether. It now calls its buying groups "Video Investment and Activation," or VIA, units. That's especially symbolic, because MediaVest, an agency that was originally named TeleVest, has its roots in TV. But …