EDIT: There are 10 000 darth vader’s randomly inserted into LEGO Star Wars Sets in USA and a lot bigger amount in the world such as Europe, Germany and Austria. (thanks to russell for pointing this out)
The chrome vader is currently being given out by ToysRUs in germany and austria when you spend a minimum purchase of $25 Euro. You can also purchase them from ToysRUs for only $5 Euro. There have also been rumours that UK ToysRUs will soon receive this offer as well.
It seems like the chrome vader may not be that special after all considering they are now for sale instead of randomly getting one in a set.