The Ran explains that R' Nosson's statement that a neder is tantamount to building a bama, and keeping the neder rather than being sho'el on it is like sacrificing at a time of issur bamos, is limited to nedarim on a d'var ha'reshus. But, a neder on a d'var mitzvah is encouraged even according to R' nosson. Based on this, if one would make a neder on a something which is not kosher or has a shayla in kashrus, even r' nosson would agree that there is no mitzvah to be sho'el on such a neder. Since the whole source for neder being yeish lo matirin and therefore not batul, is R' nosson, it comes out that if one were to make a neder to prevent eating a chashash issur, that neder would be ein lo matirin since there isn't a mitzvah to be sho'el and therefore would be batul. [Regarding the issue whether an item with 2 issurim, one ein lo matirin and one yeish lo matirin is batul, would seemingly be dependent on the machlokes rashi vs. Ran by yeish lo matirin. Acc. to the Ran it has issur status because of the ein lo matirin and therefore should be batul, but acc. to rashi since he can avoid the issur that is yeish lo matirin and eat it b'heter much'lat at a later time, he should wait so it isn't batul].