Question

which is a vague pronoun and why? this increasing pollution is a big problem or this is a big problem

Answer 1

A vague pronoun means that it's not clear to what or whom it refers.

I assume this is your pair of sentences:

"25a. Air pollution normally increases along with population growth. This increasing pollution is a big problem for many cities.
25b. Air pollution normally increases along with population growth. This is a big problem for many cities."

In 25a, "this" could refer to population growth which is clearly not the intention.
25b clarifies this confusion.

Answer 2

In a senior moment, I reversed these two sentences.

25a does not have a vague pronoun.