Consider the genre of music that you will be dealing with when going to hire a vocalist. You do not want to just hire someone to perform and then realize they sing country when you were looking for someone to perform with a rock and roll background. Make sure you look into it and not just assume.
Consider the style of training that the vocalist has had when looking to find someone to perform for you. You will find that there are many different styles and training levels out there. Make sure to find the one you are truly after. You do not want someone who is classically trained performing for a group of young kids.