Gemara

7b (Rav Gidal): "Nishbati va'Akaymenu Lishmor Mishpetei Tzidkecha" teaches that one may swear to fulfill a Mitzvah.

Question: The oath (of Kabalas ha'Torah) of Sinai already obligates him!

Answer: He teaches that it is proper to do so.

9a (Mishnah): If one said "like the vows of Resha'im, I am a Nazir (or I must bring a Korban...)", the Nezirus or vow or oath takes effect;

If he said "like Nedarim of Tzadikim" it has no effect. If he said "like their Nedavos (vows without responsibility to compensate if the animal gets lost)", his Neder of Nezirus or a Korban takes effect.

Question: Our Tana distinguishes between Nedarim and Nedavos. This is not like R. Meir, nor like R. Yehudah!

(Beraisa - R. Meir): "It is better not to vow (than to vow and not pay)." Better than both of these, is not to vow at all;

(R. Yehudah): It is better to vow and fulfill it.

Answer #1: Our Mishnah is like R. Meir. In the Beraisa he explains the verse, which discusses only Nedarim, but not Nedavos.

Question: Surely, it is better not to take a Neder lest one transgress. The same concern applies to Nedavos (perhaps he will benefit from it)!

Answer: He can do like Hillel, without concern;

(Beraisa): No one ever transgressed Me'ilah with Hillel's Olah. He used to bring it to the Mikdash while it was Chulin, be Makdish it, and immediately offer it.

10a - Answer #2: The Mishnah is like R. Yehudah. He meant that a Nedavah that is fulfilled is better than not vowing at all. He did not say so about a Neder.

We must emend R. Yehudah's opinion in the Beraisa to say "better... is to be Nodev and fulfill it." He holds that one should be Makdish his animal in the Mikdash, just before offering it (like Hillel).

A Nedavah of Nezirus is like R. Yehudah taught:

(Beraisa - R. Yehudah): Early Chasidim never needed to bring a Chatas for sin. They would accept a Nedavah of Nezirus, in order to bring a Chatas.

9b (Beraisa - Shimon ha'Tzadik): The only time I ever ate from an Asham Nazir Tamei was of a man with beautiful hair. I asked why he became a Nazir (which mandates cutting off his hair at the end). ii. The Nazir: I saw my reflection, and my Yetzer was obsessed with my beauty. It threatened to destroy me! I countered 'do not pride yourself with a world which is not yours. I will become a Nazir, and shave off this hair to Hash-m!' iii. Shimon ha'Tzadik kissed him. 'There should be more Nezirim like you. You exemplify "one who will vow to be a Nazir to Hash-m"!'

Rishonim

Rif and Rosh (1:8): We conclude that our Mishnah is R. Yehudah. A Nedavah is better than not vowing at all. We must emend his opinion in the Beraisa to say "better... is to be Nodev and fulfill it." R. Yehudah holds that one is Makdish his animal immediately before offering it, so there is no concern lest he benefit from it.

Rosh: If one is obligated to do a Mitzvah with his body, e.g. to learn a Masechta, and he fears lest he slacken and it is in his power to fulfill it, he may vow to invigorate himself, like Rav Gidal taught. All the more so, if one often forgets and transgresses a Lav or slackens from fulfilling an Aseh, it is permitted and a Mitzvah to vow or swear to be stringent on himself to fulfill Mitzvos. About this we learned "Nedarim are a fence for Prishus" (Avos 3:13).

Rambam (Hilchos Nedarim 1:25): If Levi said to David "it is Alai like Nedavos of Kesherim if I will eat from you", and he ate, he is liable. This is because Kesherim vow Nedavos of Nazir and Korban.

Kesef Mishneh: A vow "k'Nidvos Resha'im" does not take effect, for if one makes a Nedavah, he is not a Rasha in this matter (Ran, from Tosefta 1:1).

Rambam (13:23): If one vowed to correct his mindset and fix his deeds, he is zealous and praiseworthy. E.g. a glutton may forbid meat to himself for a year or two. One who blundered with wine may forbid wine to himself for a long time, or to ever get drunk. One who fervently pursued bribes may forbid himself to take gifts or to benefit from people of his country. One who was haughty about his appearance may vow to be a Nazir. Vows like this are Avodas Hash-m. About them Chachamim said "Nedarim are a fence for Prishus."

Rambam (25): It is a Mitzvah to fulfill Nedarim of Hekdesh. One should permit them only in pressed circumstances - "Nedarai la'Shem Ashalem."

Poskim

Shulchan Aruch (YD 203:3): One should fulfill Nedarim of Hekdesh, and permit them only in pressed circumstances.

Source (SMaG, cited in Beis Yosef DH Kosav SMaG): We learn from "Nedarai la'Shem Ashalem." Nedarim are a fence for Prishus when it is something he can fulfill, e.g. 'I will not drink wine today if I do not do this Mitzvah in the morning', or 'I will not eat until I finish this Halachah.'

Source (Gra 3): The Ran (59a DH Terumah) says that one may not permit Nidrei Hekdesh.

Shulchan Aruch (4): One must be careful not to vow anything, even to give Tzedakah. If he has the money, he should give it immediately. If not, he should not vow until he attains the money.

Beis Yosef (DH Al): The Tur cites the Rosh to say that he should not be Noder until he attains the money. He means, to be Nodev.

Shulchan Aruch (5): At a time of distress it is permitted to vow.

Source (Beis Yosef (DH Kosvu b'Tosfos): Tosfos (Chulin 2b DH Aval) cites Bereishis Rabah (70:1), which learns from "va'Yidar Yakov Neder."

Shulchan Aruch (6): If one says 'I will learn this Perek', and he fears lest he slacken, he may vow to invigorate himself. Similarly, if one fears lest his Yetzer seize him and he will transgress a Lav or slacken from fulfilling an Aseh, it is a Mitzvah to swear or vow to make himself zealous.

Shach (6): Merely saying 'I will learn this Perek' is a Neder!

Shulchan Aruch (7): If one vowed to correct his mindset and fix his deeds, he is zealous and praiseworthy. E.g. a glutton may forbid meat to himself for a year or two, or one who blundered with wine may forbid wine to himself for a long time, or to ever get drunk. One who fervently pursued bribes may forbid himself to take gifts or to benefit from people of his country. One who was haughty about his appearance may vow to be a Nazir. Vows like this are Avodas Hash-m. About them Chachamim said "Nedarim are a fence for Prishus." Nevertheless, one should not take too many vows of Isur. He should refrain from such things without a Neder.