It means somebody that is capable of being hurt or wounded very easily.
- Do not get yourself in a vulnerable position or else there would be people out to take advantage of you. http://sentencehouse.com/use-tacit-in-a-sentence/
- If you go out in the sun, apply your sunscreen so as to not be vulnerable to the harsh effects of the sun.
- It was without saying that the castle created by the Dutch was vulnerable on all fronts and fell down very easily during battle.
- When a con man starts looking for a target, he would always try to look out for some of the most vulnerable people that he could lay his eyes on.
- When children have vulnerable immune systems, they are bound to find themselves getting sick very frequently.
- When a mouse gets out of its shelter at night, it remains vulnerable to the owl.
- The security guard seems vulnerable as he does not possess a firearm, neither does he have any kind of stick.