First of all the Nadhr if it is beyond the capacity of a person then it is invalid from the beginning. Therefore, the nadhr of Salaat if above your capacity then it was not engaged.
The nadhr can be erased by paying the penalty of either feeding 10 poor or clothing them.
As for the sin , u tried to avoid it with the best of your capacity and a slip happened, in such cases those amazing believers feel defeated by the Shaytan after holding them selves for quite a time. So don’t worry seek repentance Allah swt is seeing your efforts to avoid and He loves it. Do not give up, be ready for the next strike of the Shaytan. Keep the practice of reciting the last chapter in the quran which keeps the Shaytan away. Try it! when ever u encounter bad thoughts from Shaytan just recite it and u will see the miraculous effects of it.