As an actual Greek person, the YON-ISS pronunciation the person above me uses is strange. Are they Texan or something? (nothing on Texans, just wondering, because they do have a particular accent.) I've always used and heard the pronunciation Yia-Nis with the I in the Nis making an ee sound.
Someone in my school has this name, but they go by Yianni (YON-EE) instead.