The word, “vain” means “useless” or “empty.” It means “hollow” or “unsuccessful.” When Jesus used Isaiah’s words to criticize the people of His day, He wasn’t criticizing the way they were praying or singing. He was criticizing their hearts and the way they were living.
He went on to criticize the way they kept the custom of “Corban,” but disobeyed God’s command to financially provide for their parents. He said, “You have a fine way of rejecting the commandment of God in order to establish your tradition” (Mark 7:9).
I would certainly agree that we should worship God the way He says for us to worship and we should not invent unauthorized ways of worshiping. In fact, I could not agree with that more. But if we think avoiding “vain worship” is done by simply worshiping according to the rules, then there is a very good chance that we ourselves are truly guilty of vain worship.