Someone from the Lake HS Band Student Body Vibrato does not harm voice at all. Vibrato is caused when the muscles that stabilize the larynx are relaxed, permitting the voice box to move a little. Stopping this natural process uses and strains muscles. With wind instruments, vibrato DOES result from variance in pitch, not volume, like you foolishly stated. Comparing strings to elastic is a faulty comparison at best.
New strings go flat many times faster than old ones.
Old strings, when broken in, can hold a desent tuing for months. The sound of strings deteriorates due to build up of dirt on strings. Boiling strings in water will revitilizes sound. Vibrato does not induce intonation problems. Intonation is depentant on control of one's voice and vocal muscle strength.