What is Vector Normalization? How is that useful? 

If the documents in the corpus are of varying sizes, the larger documents are more likely to have higher word counts across the vocabulary simply due to them containing more words. In that case, normalization can scale the word counts to a more even level across all documents. Techniques for normalizing text vectors include L2 normalization or dividing by the number of tokens in the document, which roughly corresponds to the rate of occurrence of a given token in a document.