They asked me in the shiur concerning the safek where one says "zeh
k'zeh" and the doubt is whether maikara matfis or b'hetera matfis. Why don't we just ask him what he meant? The answer is probably that it is too late to ask, the neder is already chal, but I couldn't find anyone that explains this concept.
Every Neder needs "Piv v'Libo Shavin," one's words have to be consistent with one's thoughts. If one does not want in his heart to make a Neder, then the Neder will not take effect. The question here, and in all the other cases throughout the Perek, is what do the words that he said mean. If the obvious meaning of his words is to be Matfis b'Hetera, then even if he wanted a Neder, it will not take effect.