"Legal vacuum" and "legal void" describe the same circumstance: the absence of any law which addresses an issue. A "legal loophole" is different. "Legal loophole" presupposes that a law exists with regard to a subject, but there is an ambiguity or omission in the law which allows a person to escape the intent of the law. Here's an example of such a loophole: You want to build a store that contains 800 square meters. However, the law says that a person may not construct a store larger than 500 square meters. So what do you do? You build two stores next door to each other, each one containing 400 square meters. The loophole (omission) in the law is that it doesn't address the case of a person constructing more than one building in the same area.