Answer:
If a vague pronoun is used in a sentence, it means that we cannot clearly determine what or whom the pronoun refers to. Pronouns that can be vague in certain contexts are: it, that, this or which. In this sentence 'I don't understand why it won’t fit in my backpack.', the vague pronoun is it, since we are not told (and cannot guess from the context) what 'it' refers to.