Answer: Someone
Step-by-step explanation:
In the sentence;
"someone should offer students cash bonuses for high ACT and SAT scores",
the vague word is "someone".
Vague words refer to words that are not specific or precise. They are used usually when one is not sure of the right words to use, to make a sentence short or to give a reader an opportunity to explore and figure things out.
By using "someone" in the sentence above, the writer is being vague and does not want to be specific probably because he is unaware of the person or body that should pay students for scoring high in ACT and SAT or he just wants his readers to figure out who or he just does not want to point fingers.