What should one do who vowed to stop watching pornography if he was accepted into university, then returned to watching it after a period?

You must stop watching forbidden films and repent to Allah Almighty. Watching them inherits darkness in the heart, tightness in the chest, and the dominance of Satan. Your statement is not a vow unless you intended it as such. In that case, you must stop watching films for two reasons: from the perspective of Islamic law and from the perspective of the vow. Scholars have differed on the ruling concerning vowing to do something that is already obligatory by original Islamic law.