I am looking to create a viral video using a voice impersonator as the main voiceover being a clear imitation of a real life celebrity. I am wondering what the legal issues with this type of thing may be.
Where is the parody part? Copying a celebrity voice, if truly a parody, might be allowed. Merely copying a celebrity voice in video and having people think it is the celebrity could get you in a lot of trouble. Tread carefully. http://en.wikipedia.org/wiki/Hustler_Magazine_v._Falwell
Interesting issue. If you infringe on somebody's rights and they come after you, it might end up being the best thing that happens to you because of all the publicity. I have seen things monumentally backfire on people who did nothing wrong but try to enforce their own property rights. It is a strange country we live in. But, to get to your point, what you are looking for is a legal opinion on a matter of "FAIR USE". What you are proposing could very well be within the bounds of fair use. You go go ahead and make the product then show it to a lawyer. You could also show it to the actual of the celebrity and see what they say. A lot is going to depend on your intent. If you are intending to do this in order to provide entertainment/amuse a lot of people, it will be easier for than if your intent is to blatantly make money.