He didn't say anything about negative ads that will "start soon"...the context was he was talking about how Sanders and Clinton perform in general election polls and he noted Clinton has already been the subject of a lot of negative advertising and Sanders has not. It seems that what he was saying was that if Sanders were ever to be the nominee he would be 'redefined' by negative advertising and his general election numbers would go down.
Yeah, talk about misleading. Vilsack is simply stating the FACT that Sanders' favorability/GE numbers are so good because Republicans have not bothered to attack him at all, short of a few cheap socialist jokes. And that if he actually won the nomination, he'd be receiving those attacks in abundance, all the while being outspent about 10:1.
