Why did Yaakov vow? "Tov Asher Lo Sidor" (Koheles 5:4)!
Moshav Zekenim (from Bereishis Rabah 70a): One may vow at a time of distress. 1
Tosfos ha'Shalem (2): Since he did not have anything to offer now 2 [he could only vow].
Why did Yaakov stipulate "Im... (if Hashem will guard me)"? Hashem already promised him! Also, one should not serve Hashem on condition to receive reward!
Rosh, Da'as Zekenim, Hadar Zekenim #1, Moshav Zekenim: This was a not a Tenai, rather, a Shevu'ah. When Hashem will do... I will....
Hadar Zekenim #2: He meant, if Hashem will guard me from idolatry, murder and Arayos....
Tosfos ha'Shalem (5): Hashem's promise was in a dream, and every dream has Batel matters....
Tosfos ha'Shalem (6): He stipulated, if the promise will be fulfilled immediately. "Lechem Le'echol" refers to a wife. 1
Tosfos ha'Shalem (9): It is difficult to say that he was concerned lest sin nullify the promises. 2 Rather, he was concerned lest his dream was due to his thoughts during the day, that Yitzchak gave to him Birkas Avraham.
Tosfos ha'Shalem (11, citing R. Yehudah ha'Chasid, citing R. Yitzchak of Rusiya): Yaakov cannot manage with less than [minimal food and clothing]. He vowed to tithe only what he receives above this.
Riva (to 28:22, citing R. Tam): Yaakov accepted to fulfill whether or not Hashem will give to him.
Ohr ha'Chayim #1: Here, "Im" is surely, and not a Safek, like the verse "Im Kesef Talveh" (See Rashi to Shemos 22:24).
Ohr ha'Chayim #2: "Im Yihyeh Elokim Imadi" means, if Midas ha'Din will agree. Hashem (Midas ha'Rachamim promised; sometimes Midas ha'Din blocks it).
Malbim: Yaakov stipulated also (in 28:21) "v'Hayah Hashem Li LE'lokim," i.e. I will serve Hashem and remain a Tzadik. Hashem did not promise this, for it depends on free choice.
Malbim (to 28:22): If Hashem will not be Elokim to me - i.e. I will sin in Beis Lavan - all good will be for my evil, and I will not fulfill my vow.
Ha'amek Davar: Perhaps the promise to guard him was only until his children will be born, and that they will not die out. Perhaps the promise for food and clothing was so he will not die, but he will lack food and his clothing will be torn. He now asked for food and clothing according to Hashem's ability to give.
Why did Yaakov stipulate about a wife, but not about children? His primary desire was to have children! (Refer to 31:17:1.1:1 - PF.)
Why is this difficult? Berachos 4a asks why Yaakov feared Esav in spite of Hashem's promise, and says that he was concerned for sin! (see Rashi to 32:11. In fact, Gur Aryeh writes that this was indeed was Yaakov's fear also now; perhaps sin might hold them back, or his merits might prove insufficient.) According to Tosfos, perhaps in that case he found himself in a dangerous situation, or he was worried lest he had sinned through marrying two sisters, by getting angry at Rachel and Lavan, by benefiting from Lavan's money.... (PF)
Why did Yaakov stipulate about clothing? Hashem did not promise this!
Riva #1, Hadar Zekenim: Clothing to guard from heat and cold is part of Shemirah.
Tosfos ha'Shalem (14, citing Maharam bar Baruch): He requested Bigdei Kehunah, i.e. the fulfillment of the purchase of the Bechorah.
Why does it say "Leimor"?
Hadar Zekenim, Alshich (from Bereishis Rabah 70:1): This teaches that for generations, Yisrael will vow at times of their affliction. This is why vows are attributed to him - "... Nadar la'Avir Yaakov" (Tehilim 132:2). 1
Ohr ha'Chayim: A vow must be spoken. If it is in the heart, it is not a vow. 2
Shulchan Aruch (CM 212:8): Some say that if one resolved in his heart to give to Tzedakah, he is obligated. (We learn from "v'Chol Nediv Lev Olos" (Divrei ha'Yamim II 29:31)). Some disagree; we do not learn Chulin from Kodshim. Perhaps all would agree here, for he was not Makdish a particular item. However, some say that a vow in the heart to fast takes effect (see Taz OC 562:8). (PF)
QUESTIONS ON RASHI
Rashi writes: "'If Hashem shall be with me' - If He will keep these promises to me." What prompted Rashi to explain this way?
Gur Aryeh: We cannot explain it as a stipulation, for Hashem had already assured Yaakov of all this (28:15). Yaakov meant, 'if Hashem will keep that assurance.' Since there is no reason that Yaakov would single out this term of the promise in particular, Rashi explains all the following phrases as also referring to the promises mentioned above.
Rashi writes: "If He will keep these promises to me." Why was Yaakov worried that sins might withhold their fulfillment? The Gemara (Berachos 7a) states that Hashem does not renege on His word to bestow good?
Gur Aryeh (to 32:8): There is a fundamental difference between an assurance (Havtachah) and a prophecy (Nevu'ah). An assurance can be changed because it is dependent on the recipient's merit. A prophecy, however ,is Hashem's decree, it is independently true and will be fulfilled. This is why prophecy is usually related in the past tense, it is already a reality. 1
Rashi writes: "One who must search for bread is called 'Ne'ezav' (abandoned)." Why is this so?
Gur Aryeh: Bread is a constant necessity for a person (unlike other needs that arise occasionally). One who lacks this need is completely abandoned.