Gemara

(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;

9a (Mishnah): 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.

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 "it is better to be Nodev and fulfill it."

20a (Beraisa): One should not regularly make Nedarim, for he will eventually transgress Shevuos.

22a (R. Yanai): A vow causes the Heavenly Beis Din to open one's case and scrutinize his deeds.

(R. Aba): He learns from "after Nedarim Levaker (to investigate)".

(R. Gamliel): "One can damage like a sword through his utterance, but the words of Chachamim heal" - anyone who vows should be pierced by a sword, but Chachamim can cure him (by annulling the vow).

(R. Noson): One who vows is like one who builds a Bamah (private Mizbe'ach). One who fulfills his vow (he did not permit it) is like one who offers a Korban on a Bamah.

(Shmuel): Even though he fulfills the vow, he is called a Rasha.

(R. Avahu): We learn from a Gezeirah Shavah between "when you will cease from vowing, there will not be sin in you" and "the wicked ceased from rage."

(Rav Yosef): A Mishnah teaches that one who vows is called a Rasha.

(Mishnah): If one vowed 'like vows of Kesherim", this is not a vow. If he vowed 'like vows of Resha'im', this is a vow...

77b - Rava: A Chacham came from Eretz Yisrael, and said that Chachamim permitted a vow for the son of Rav Huna bar Avin. They told him to pray for forgiveness for having vowed.

(Rav Dimi): If one vowed, even if he fulfills it, he is called a sinner.

(Rav Zvid): We learn from "When you will refrain from vowing, you will not have sin." This implies that if you will vow, you will have sin.

Rishonim

Rif and Rosh (1:8): One should not take a Neder, lest he transgress it.

Rosh: A Kosher person should not vow or swear about anything. If one has money to give to Aniyim or redeem captives or such matters, he should give it immediately. If not, he should be quiet until he attains the money. If people are pledging Tzedakah and he needs to pledge with them, he should say 'Bli Neder.'

Rambam (Hilchos Nedarim 1:25): If Levi said to David "it is Alai like Nidrei Resha'im if I will eat from you", and he ate, he is liable. Nidrei Resha'im include Nezirus, Korban and Shevuah.

Kesef Mishneh: "Nidrei Resha'im" takes effect, for they are Resha'im in this matter (Ran, based on Tosefta 1:1).

Rambam (26): If he said "like Nedarim of Kesherim" it has no effect, for Kesherim do not vow to forbid things amidst anger.

Rambam (13:23): One may vow to correct his mindset and fix his deeds. E.g. a glutton may forbid meat to himself for a year or two...

Rambam (24): Nevertheless, one should not take too many vows of Isur, nor accustom himself to them. Rather, he should refrain from things from which he should refrain, without a Neder.

Rambam (25): One who vows is like one who built a Bamah when this is forbidden. If he transgressed and vowed, he should ask to permit it, lest he stumble in it. This refers to vows of Isur.

Poskim

Shulchan Aruch (YD 203:1): Do not take Nedarim regularly. Anyone who vows, even if he fulfills it, is called a Rasha and a sinner.

Taz (1): If he is a Rasha, we need not say that he is a sinner! It seems that he is a Rasha if he has a Pesach to permit it, but does not do so. If not, he is only a sinner.

Beis Yosef (DH Tanya l'Olam): One should not regularly make Nedarim, for he will eventually transgress Shevuos, which are more severe. Some texts say 'he will eventually transgress Nedarim.'

Beis Yosef (DH Yerushalmi): 22a (R. Yanai): The sin of vowing causes the Heavenly Beis Din to open one's case. Alternatively, it is because he considers himself to be a Tzadik, and is confident that he will not stumble.

Shulchan Aruch (3): One who vows is like one who built a Bamah when this is forbidden. One who fulfills it is like one who offered a Korban on it. He should have permitted his vow.

Beis Yosef (DH Tanya): Rashi explains that he is like one who built a Bamah to idolatry. The Yerushalmi supports him. The Rosh and Ran say that he is like one who built a Bamah (to Hash-m) when this is forbidden. He thinks that he does a Mitzvah. He forbids himself, just like the Torah did. This is like bringing a Korban that Hash-m does not desire, e.g. outside the Mikdash.

Shulchan Aruch (4): One must be careful not to vow anything. Even to give Tzedakah, it is not good. If he has the money, he should give it immediately. If not, he should not vow until he attains the money. If people are pledging Tzedakah and he needs to pledge with them, he should say 'Bli Neder.'

Shach (4): He must explicitly say 'Bli Neder.' A Stam pledge to Tzedakah is a Neder.

Beis Yosef (DH Al): The Tur says not to vow until doing the matter. It seems that he discourages vows to do any action. Or, he says not to vow to do any action until he is ready to do it immediately, just like Hillel was Makdish his Olah right before offering it. However, the Rosh established the Mishnah like R. Yehudah, who permits a Nedavah right before offering it, but not a Neder. The Tur did not distinguish. Therefore, the first Perush is primary. The Tur did not permit a Nedavah right before fulfilling it, for R. Yehudah permits only regarding Korbanos, in which one must be Makdish it before offering it. One need not be Makdish Tzedakah. We can defend the second Perush. One about to do a Mitzvah will surely say 'this is', not 'it is upon me', so the Tur did not need to specify that he be Nodev, not Noder. The Tur says in the name of Rosh not to be Noder until he attains the money. He means, to be Nodev.

Taz (4): The Bach (DH Al) explains that one should not vow about what he will do when he gets something. This does not explain why the Tur said 'until'.

Taz (5): We permit vows about Mitzvos he will do with his body, but not a vow to give Tzedakah. There is a bigger Yetzer Ra not to give Tzedakah, so we fear lest he transgress. Some say that we are concerned lest he lack money to give, or lest the Oni not accept. This is wrong. If so, he is exempt!

Shulchan Aruch (7): Vows to correct his mindset and fix his deeds are Avodas Hash-m. Nevertheless, one should not take too many vows of Isur, nor accustom himself to them. Rather, he should refrain from improper things, without a Neder.