Is delaying wearing the niqab, after intending to wear it at the beginning of university, considered a sin or a vow?

A vow has a specific form; it is not considered a vow unless it is verbally pronounced. Thus, whoever intends to make a vow but does not verbalize it is not obligated to fulfill what was intended, let alone if there was no intention of a vow at all. The preponderant opinion among scholars is that covering a woman's face is obligatory, so a Muslim woman must adhere to it unless she fears harm.