In the sugya of תחילתו בפשיעה וסופו באונס, the Nemukei Yosef (Baba Kama 10b b'dafei HaRif) writes in the name of the Ramah that if the person isn't negligent originally because he had not reason for the o'nes situation arising in the end, they are not responsible. The Nemukei Yosef continues, therefore one who doesn't daven when they could have, but later an unexpected o'nes arises that prevents them for davening, it is considered an o'nes and they have the right to daven a make-up tefillah. Similarly, one who makes an oath to pay a debt by a certain day, although he had money earlier from which to pay, if by the time the debt is due he is an o'nes and is unable to pay, he is considered an o'nes and not in violation of his promise. This issue is actually a debate in the Rishonim. Although the Rama in Y.D. 232:12 cites an argument in the context of an oath as to whether we consider it o'nes, the Shluchan Aruch in Hilchos Tefillah (108:8) paskens clearly like the Nimukei Yosef that it is regarded as an o'nes, and the Rama there makes no mention of any dispute.
The gemara says that children die for their father not fulfilling their pledges of tzedaka and korbanos. Rav Moshe (Dibros 89) asks that presumably it is speaking about a situation where the father doesn't have money to pay so he is an o'nes. Why then are the children punished?
Rav Moshe suggests that this would actually be dependent on the machlokes mentioned above whether pushing something off that later develops into an o'nes qualifies as o'nes, and suggests that our gemara is against the Nimukei Yosef. The difficulty with this approach is that the Nimukei Yosef is not just a da'as yachid, but we actually pasken like the Nimukei Yosef.
To me it seems that this gemara has nothing to do with the Nimukei Yosef. We are not speaking about a situation where the person made a pledge when he had money, and later lost the money. That would qualify as an o'nes as the Nimukei Yosef says and there would be no punishment. Rather, we are speaking about one who is somewhat negligent in not properly calculating what they can really afford, leading them to pledge what they cannot afford at the time they make the pledge. This is on the level of sho'geig but not an o'nes, therefore one can be punished for that. This approach is explicit in rashi d.h. ki shegaga - בשוגג קפצתי לנדור ולא אשלם. The situation is at the time the person made the pledge they were too hasty and didn't properly calculate to see that they can't afford what they are pledging. For this negligence they are responsible.