What is a vague pronoun?

A vague pronoun is a pronoun for which it's not clear which noun is its antecedent. In other Words, you can't tell which Word the pronoun is replacing.

Example 1:

When the car hit a tree, it made a terrible noise. Did the car make the noise or did the tree make the noise? The sentence is Worded in a way that it's not clear.

A better form of this sentences is:

The car made a terrible noise when it hit the tree. Here you can tell that the pronoun 'it' is referring to the antecedent 'car'.

Example 2:

*The literal reference in the above example is neither the car nor the tree: it is the action of the car hitting the tree. There is no real vague pronoun. However, there is in:

Bill went with John to his father's store. (vague)

To rewrite it:

Bill went to his father's store with John.

Bill and John went to the store owned by Bill's father.