Im not brushed up on copyright laws, but Im pretty sure its not illegal to create something that "looks" like "something" as long as its not a trademark, logo etc..
The bulk of vbulletin styles are nothing more than color codes in the first place; and im pretty sure you cant copyright that
What is this style if you want to get specific.. blue with transparent clouds and some line work; this is not the first of its nature and its not unique.. ive seen many like it and actually have got an opinion from many graphics artist saying the design of this theme was copied from another site..
Now im not liberty to say whether it is or not, I dont care.. im not a graphics person but Im pretty sure it would not be against the law for this guy to create a theme based off his original work with inspiration from another work.
Regardless if you or vbulletin.org give him permission or not.