What is the ruling on vowing to pray 20,000 rak'ahs for someone who vowed to pray 10,000 rak'ahs upon achieving a total score exceeding 75 in two exams, but the purpose of the vow, which was passing the academic stage, was not fulfilled?

There is no obligation to fulfill this vow because vows and oaths are interpreted according to the intention of the one making the vow, as long as the intention is consistent with the wording. Since you intended to achieve the grade along with surpassing the overall GPA, and this did not happen, nothing is binding upon you. This is supported by the Prophet's (peace be upon him) saying: "Deeds are only by intentions, and every person will have what he intended."