Is a single expiation required to cancel the vow, or is expiation required for each time the vow was not fulfilled? And is it permissible to fast instead of feeding in expiation for the vow?

If you utter a phrase that implies repetition, then you owe an expiation for each time you broke your oath. If you have surplus wealth beyond your basic needs that suffices to feed the poor, then expiation by fasting is not permissible. Using oaths to abandon sins is an unsound approach; the straight path is to seek Allah's help and offer sincere repentance.