What does it mean when there is no ceiling or "CLR" in an airport METAR?
Vertical Visibility with a number means that cloud layers cannot be determined because of haze, fog or mist, instead it's a measure of how far up the weather station can see. Because its sensors cannot penetrate further the station cannot tell you whether there's clouds above that level or not, but it does say in this case that there are no clouds below 1000 feet. 

You may see VV/// sometimes, this means cloud cannot be measured. If fog, mist or haze and poor visibility is reported as well then it's likely that the fog is so thick the sensors cannot penetrate it, if on the other hand visibility is good with no fog or haze it's likely a sensor malfunction.