How many times have you come across people who profess to be Christians, but act totally opposite of Christ-like? THAT is taking the title of Christianity in vain.
Are you a child of God? "Yes, I am a child of God!" Then, why do you encourage the things that God speaks against? Things like abortion, pornography, drugs...
How many times have you "joined the crowd" that belittles God's name or place in society, yet use God's name to uphold your status in the same society?
Sure, using God in a curse-word is vain, also.
According to Bing, the word "vain" means something that is useless, coming to no result, having no meaning, pointless. It can also mean narcissistic in nature. All of those meanings are at work in this one verse.
"With God as my witness, I WILL get revenge!" Vain.
"How dare you! I'm a CHRISTIAN!" Vain
"That God-blessed thing cut my finger!" VAIN.
God's name is HOLY. It should only be used when speaking a truth (God cannot lie). It should only be used in the context of speaking of someone who is pure, unchanging, stable, solid and HONEST.
And, if you call yourself a CHRISTIAN, you should ACT like it!