Nedarim 16b - Shavua and Neder to be mevatel a Mitvah

The gemara says that one can make a neder to be a mitzvah since it doesn't directly go against the Torah obligation (Torah obligation is on the person and he is placing issur on the object), but one cannot make a shavua to violate a mitzvah since he going head on with a torah obligation. By shavua it makes no difference whether he is trying to make a shavua on an aseh (Shavua not to eat matza) or on a lo t'aseh (shavua not to eat pork) - the shavua will not be binding (unless it is made b'kollel in which case there is a lot of discussion in Rishonim). But by neder, it may only be binding if he makes a neder to be mevatel an aseh, but if he makes a neder on a lo ta'aseh (neder to assur pork), it is a big machlokes Rishonim (brought in Ran in shavuos and nimukei yosef here) whether such a neder is binding. The Ramban holds that it would be binding since the issur pork is an issur gavra and one can place on that an issur cheftzah. Perhaps those who argue with the Ramban would consider issurim such as pork to be both an issur gavra and an issur cheftzah so that both shavua and neder will not be chal on it.
Another issue is when one makes a shavua to be mevatel a d'rabonon mitzvah, is the shavua binding? The Shulchan Aruch in Hilchos Ta'anis (570) rules explicitly that the shavua would be binding since the mitzvah is only d'rabonon, a d'orayasa shavua can be chal on it and it would be forbidden to violate the d'oraysa shavua in order to perform the d'rabonon mitzvah (we saw in the Ran on 15b that even when a neder is binding d'rabonon and one has a d'oraysay mitzvah, he should forgo the d'oraysa mitzvah to maintain the d'rabonon neder since chazal are oker davar min hatorah b'shev v'al ta'aseh, certainly one must maintain a d'oraysa shavua and forgo on a d'rabonon mitzvah).