I think it's alright to use the word vultures, especially if the piece starts out more homorhythmic, with a somewhat thinner texture so the words are more transparent initially. If the audience is provided with the text, that would make it even clearer...but I don't think it will sound too muddy the way it is. Besides, off the top of my head I can't think of any other word with the same connotation, so if vultures is what you really mean to say, don't change it. Perhaps you could have a read-through to see how vocalists would feel about it musically? I know it won't have the same sound as a full choir, but even getting four people for a simple SATB reading could provide you with some good feedback.