All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First of all, it should be noted that a person is obliged to immediately repent to Allaah from all sins, whether minor or major, and it is forbidden for him to return to sin. Allaah Says (what means): {O you who believe! Turn to Allaah with sincere repentance!}[Quran 66:8]. 'Umar and Ibn Mas'ood interpreted sincere repentance to mean; a repentance by which one does not return to sin again. But if one sins again, he is obliged to repent again and so forth. Returning to sin should not prevent someone from repenting again. One is obliged to repent to Allaah any time he sins. As regards the narration which you mentioned in the question, please refer to the details in Fatwa 88207 about its interpretation by the scholars.
With regard to the issue of vowing and doubting the vow and its wording, then if one doubts the vow itself as to whether or not he made it, he is not obliged to do anything, and a person who doubts is not required to do anything as in principle he is innocent and doubt cannot overrule certainty. However, if he is sure that he made a vow to Allaah, then it is an obligation to fulfil the vow that he made to Allaah. Allaah Says (what means): {And fulfil the Covenant when you have covenanted, and break not the oaths after you have confirmed them.}[Quran 16:91]. If one breaks his oath, he is obliged to expiate for it; the evidence is that Allaah referred to covenants as oaths in the above verse {…and break not the oaths}. So he is obliged to expiate for each time he broke the oath. If one doubts about how many times he broke the oath, then he has to estimate a number so that he would be on the safe side.
For the expiation of breaking an oath, please refer to Fatwa 102811.
The amount of feeding a poor person is half of a Saa' (i.e. 1,240 grams as a Saa' is 2,480 grams) of barley for each poor person from the common food in the country.
Allaah Knows best.