In my opinion, this is the most difficult achievement in the game. Basically, to get a technique medal, you must get a specific amount of vibrato bonus points. What I did to achieve that was singing louder than usual, while punching my chest lightly and quickly (you must not punch too hard!). The impact on your chest will make the air flow through your throat and vocal cords faster, allowing you to use the vibrato from the beginning to the end of the song.