Make sure that when you are hiring a vocalist that they are able to perform all of the songs that you would like performed. But also keep in mind that you want them to perform the songs in the style that you want as well. It is a matter of style that you are concerned with here.
Vocalists are what make the song sound how it is. Each different vocalist can put their own spin on the way it will be portrayed. Different genre songs performed by different style vocalists can very easily convey different meanings. Check into what type and style of vocalist you want.