Elvis used vibrato in every song. He couldn’t do those slow songs with excellent vibrato and without practicing vibrato every day.
Vibrato takes 60% more breath to sing.
Simple Vibrato
Same as Notes Tuning method but add vibrato to the end of the vowel so it has 3 cycles or more.
Hard Vibrato
Hold a note using a strong vibrato, almost a wobble, for 8 counts. When mastered raise the counts to 12. Then do messo forte. Then forte. Do this throughout your vocal range.