Vocals are the same as recording drums or any acoustic instrument. They sound dull when the sound Waves don't have stuff to bounce off of and mature as they travel around the room. Larger rooms can lend a lot to vocal recording, room treatment is generally advised.
Recording in a closet or something of the nature is not a good idea unless you are recording for voice over because the result will be dull and won't have much harmonic character. i.e. It will sound boxy. Think of recording in a large treated room like controlling the sound waves whereas recording in a small room/box is eliminating the sound waves.