Gemara

(Mishnah): A stringency of Nedarim over Shevuos is that if one said 'Konam Sukah, I will not make one' or similarly regarding Lulav, or Tefilin, he is forbidden. Such a Shevuah does not forbid, for one cannot swear to override Mitzvos.

(Rav Gidal): "Lo Yachel Devaro." - one may not profane his own word, but he may for the sake of Hash-m's word (an oath to transgress a Mitzvah is void).

Question: Presumably, "Neder la'Shem Lo Yachel" obligates observing a Neder against a Mitzvah. We should similarly expound "Shevuah la'Shem Lo Yachel"!

Answer #1 (Abaye): One can forbid Hana'ah (pleasure) from a Sukah to himself (a Neder). One cannot forbid himself to benefit from a Sukah (a Shevu'ah).

Objection (Rava): Mitzvos Lav Leihanos Nitnu (fulfilling a Mitzvah is not considered Hana'ah. Forbidding Hana'ah from a Sukah would not forbid the Mitzvah!)

Answer #2 (Rava): A Neder forbidding sitting in a Sukah is Chal, but a Shevuah not to sit in a Sukah does not.

Shevu'os 20a (Beraisa): If one said 'eating meat and drinking wine are forbidden to me like the day that Gedalyah ben Achikam was killed', he is forbidden.

(Shmuel): The case is, he had already taken a Neder never to eat meat or drink wine on that day.

One might have thought that since he may not eat that day (Tzom Gedalyah) even without his Neder, it is as if he was Matfis in (made something else forbidden like) an Isur, not in Davar ha'Nadur (an Isur due to a Neder). The Beraisa teaches that this is not so.

Rishonim

The Rif concludes like Rava.

Rambam (Hilchos Nedarim 3:6): A Neder is Chal (takes effect) on Mitzvos like on Reshus (optional matters). If one forbade to himself Matzah on Pesach night or sitting in a Sukah on Sukos, or taking Tefilin, these are forbidden. If he ate, sat or took, he is lashed. The same applies to all similar cases.

Radvaz: A Neder is Chal to desist from a Mitzvah of action. A Neder to eat pork is not Chal. We do not say that the Aseh "Motzei Sefasecha Tishmor" overrides a Lav, for he brought the Aseh upon himself.

Question (Lechem Mishneh): Even such a Shevu'ah would be Chal, since it is Chal on Matzah, Sukah and Tefilin when there is no Mitzvah!

Answer (Lechem Mishneh): Nedarim depend the way people speak. When people discuss eating Matzah, sitting in a Sukah, or taking Tefilin, they refer to the Mitzvah.

Rosh (2:7): Rava answers that a Neder forbids objects, e.g. a Sukah. It does not look like he uproots the Mitzvah from himself, for he did not accept anything on himself. A Shevuah forbids himself. One may not uproot his obligation to do a Mitzvah! Therefore, we expound "la'Shem" to refer to Neder, which is written earlier, and not to Shevu'ah, which is written later.

Rosh (Shevu'os 3:3): Our texts say '...because it (Tzom Gedalya) is forbidden only mid'Rabanan.' When he vows it becomes Asur mid'Oraisa, and when he is Matfis in it, "Lo Yachel" applies to the new matter. Rashi rejects this text, for even if the Isur is mid'Oraisa, Nedarim are Chal on Mitzvos. This is no reason to reject it. A Neder is Chal on an Isur mid'Oraisa and one transgresses "Lo Yachel." However, Hatfasah in it is not forbidden, for he is Matfis in the initial Isur, which is not Davar ha'Nadur. If the initial Isur was mid'Rabanan, it is considered Davar ha'Nadur.

Question: Why does a Neder forbid sitting in a Sukah? Mitzvos Lav Leihanos Nitnu!

Answer (Ran 16b DH Omar Rava): He forbade sitting, even if it is not Hana'ah. However, Stam Konam forbids only Hana'ah.

Poskim

Shulchan Aruch (YD 215:1): A Neder is Chal on Mitzvos. If one said 'Konam Sukah, I will not sit (in one), a Lulav, I will not take one', he may not sit in a Sukah or take a Lulav.

Rema: Some say that we lash him for vowing to Mevatel a Mitzvah, and permit it, and he fulfills the Mitzvah. Some say that there are no lashes for a vain vow, only for a vain oath.

Beis Yosef (DH Kosav ha'Mordechai): Avi ha'Ezri says that one who swore to transgress a Mitzvah need not permit it. However, he is lashed for a vain oath. If he vowed to transgress, he must permit it, and he is not lashed. The Mordechai found elsewhere that one is lashed also for a vain vow.

Shulchan Aruch (5): A Neder is Chal on a Mitzvah only regarding an Aseh, whether or not there is a Lav for being Mevatel it.

Source (Gra 7): The Gemara (15b) said that one can forbid to himself pleasure of Bi'ah with his wife. The Gemara did not challenge this, even though a Lav forbids diminishing Onah.

Shulchan Aruch (ibid.): We do not distinguish between an automatic Isur and one that he brought upon himself. A Neder to fulfill or Mevatel a Lav is not Chal. The same applies to an Isur he brought upon himself, e.g. if he swore not to eat a loaf and later said 'it is Alai.' If he ate it, he is liable only once, for the Shevu'ah.

Source (Rashba 4:109, cited in Beis Yosef DH ul'Inyan): A Neder is Chal on an Aseh in every case. A Neder or Shevu'ah is never Chal on a Lav. Some of my Rebbe'im say that it is not Chal on an Isur he brought upon himself, but it is Chal on an Isur that comes by itself. R. Tam agrees. I disagree.

Shulchan Aruch (6): Some say that the Neder is Chal. If he eats it, he transgresses "Lo Yachel." However, if he was Matfis something else in it, it is not forbidden, for he intends for the initial Isur, which is Davar ha'Asur. However, if something is forbidden mid'Rabanan and he forbade it through a Neder and was Matfis something else in it, it is forbidden.

Question (Bach DH v'Ika): Above, the Shulchan Aruch rules that the Neder is not Chal, like the Rashba, without a dissenting opinion. Why did he wait until here to bring the other opinion? Most hold like the Rosh, that a Neder is Chal even on an Isur mid'Oraisa.

Defense (Shach 12): It is normal to bring an opinion, and later a dissenting opinion! The Rashba brought many proofs. If the Bach rejected them, he should have said why!

Taz (7): This is really part of Sa'if 5. Printers mistakenly thought that the 'Vov' in 'v'Yesh Omrim' denoted Siman Vov (6).