It can mean being obsessed with your looks. It can also mean not having a result or wasted effort. He tried in vain to convince her to see things his way. It is vain to expend more effort to achieve that which can be done with less.
When you want a definition on Ask.com you can type the word into the same question window you typed this question in. Just use this wording: Define: vain Then click Ask and a plethora of dictionary selections will be displayed for you to choose from.