What is the obligatory amount to be given in charity for a vow accompanied by the phrase "God willing" (And by God, God willing, if I get a job, I will donate a full month's salary)? Is it the salary at the time of employment or the current salary? And is fulfilling the vow or performing Hajj a priority if the money is not sufficient for both?

The statement made by someone, "By Allah, if Allah wills, when I get a job, I will donate a full month's salary," is an oath (yameen), not a vow (nadhr). If the one who swears an oath or makes a vow qualifies it with "if Allah wills" (insha'Allah), then they do not break their oath (hanith) and are not obligated to offer expiation (kaffarah) if they do not fulfill what they said, due to the Prophet's (peace be upon him) saying: "Whoever swears an oath and says, 'If Allah wills,' then there is no breaking of the oath upon him."

If the intention behind the qualification with "if Allah wills" was to avoid obligation, then there is nothing upon him. However, if the intention was to seek blessings or to affirm the statement, then fulfillment becomes binding. Since the questioner's statement is in the form of an oath and not a vow, nothing is binding upon him.