What is a good tele-converter lens?
 

I have been looking at the Century 2.0x tele-converter lens. However, before I take the plunge I would like some other suggestions.

I want a tele-converter/telephoto lens to increase (I prefer to double) the indicated focal length with minimal vigneting while still holding about a 50% zoom range (or better). I have the VX2000. Thanks.