We live in a society where most people use their tongues carelessly. Jesus, however, makes it very plain that we must be very careful concerning what we say and how we say it lest we stand condemned before God for our careless speech. For by your words you shall be justified, and by your words you shall be condemned.
Profanity is one form of careless, sinful speech.
Something that is vain is simply that which is useless, futile, good for nothing. Most people recognize that using the Lord's name to damn people or things is sinful. However, it is a careless and profane way to speak of the Almighty God. Gossip too, is a careless and sinful use of the tongue. Put yourself in the situation of the one to be talked about and ask yourself honestly if you would want the same said about you.