Nedarim 8a - Shavuah l'vatel Mitzvah

2 points:
1. The Ran and Rosh argue whether a shavua to be mekayem a mitzvah is binding in the sense that one would be in violation of ba'al yachel for not keeping his promise. Ran understands that a shavua on a mitzvah is not binding for the purpose of a korban but is binding for the purpose of ba'al yachel. The Rosh seems to understand that it is not binding at all, which is also the opinion of the Ramban brought by R' Akiva Eiger. Based on this understanding, they also argue as to what the chiddush of R' Gidal is. The Ran understands that the shavua is binding and therefore obviously not a shavuas shav, so the chiddush is that one is encouraged to make these types of shavuos. But, according to the Rosh that the shavua is not really binding, the chiddush is simply that by making such a shavua it is not an automatic violation of "lo tisa", since it at least accomplishes a function of encouraging the person to fulfill the mitzvah.

2. The Gemara says that when one makes a vow to learn a specific mesechta, it is binding even in regard to a korban, and is not considered a vow to fulfill a mitzvah. The reason is that since the torah does not explicitly indicate an obligation to learn any more than just krias shema in the morning and evening, the vow is completely binding on anything beyond what is explicitly stated in the torah. However, the Rashash asks that since one is not obligated to learn that specific mesechta, the vow should be binding to learn a specific masechta. Actually the Ritva learns this approach to be part of what the gemara is saying - since one can fulfill there obligation with some other type of learning i.e. krias shema, or any other mesechta, therefore when he makes a vow on a specific masechta, it is fully binding.

Tosafos writes that even if one makes a vow not to learn something specific, the vow is binding. R' Moshe (Dibros Moshe heara #43) explains that Tosafos cannot be understood by saying that one is not obligated to learn all sections of Torah, because both the obligation of knowing torah and the obligation of constantly learning torah actually requires a person to learn all sections of torah every day, just that it is impossible, but there is technically an obligation on every single aspect of Torah. Therefore, Tosafos can only be explained like the Ran that the obligation to learn every section of Torah is not stated explicitly. Based on this, the Rosh and Ramban would hold that a vow not to learn even a specific or obscure section of Torah on any particular day would not be binding at all.