1) Its not a real search engine, its just a meta search engine, IE, compositing results from other, real search engines. Also, all the engines it relies on use meta-tags, as opposed to Google's string search.
2) The results are laughable. When searching for "massassi": Google: 32,100. Vivisimo: 124.
3) Its slow. Google responds with results in less than a second, while Vivisimo takes several seconds.
4) The interface sucks. Google has a very slim and streamlined interface, while Vivisimo's is clunky and bloated.
And besides, why does everyone keep looking for a replacement for Google? How often is it that you find a company who's slogan is "Do no harm"? Google is more than a search engine, its a tool. Hell, its a whole suite of tools. Accept it as such, and move on.
And when the moment is right, I'm gonna fly a kite.