Why did Yakov vow? "Tov Asher Lo Sidor"!
Moshav Zekenim (from Bereishis Rabah 70a): One may vow at a time of distress.
Tosfos ha'Shalem (2): Since he did not have anything to offer now 1 [he could only vow].
Even though he did pour oil on it, he did not consider this enough. Perhaps it was less than a Log, the smallest Nedavah of oil. (PF)
Why did Yakov 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): Yakov cannot manage with less than [minimal food and clothing]. He vowed to tithe only what he receives above this.
Riva (verse 22, citing R. Tam): Yakov accepted to fulfill whether or not Hashem will give to him.
Ohr ha'Chayim #1: Here, "Im" is surely, and not a Safek, like "Im Kesef Talveh."
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: Yakov stipulated also (verse 21) "v'Hayah Hashem Li lEi'lokim", i.e. I will serve Hashem and remain a Tzadik. Hashem did not promise this, for it depends on free choice.
Malbim (22): If Hashem will not be Elokim to me
Why did Yakov stipulate about a wife, but not about children? His primary desire was to have children! (Refer to 31:17:1:1 - PF.)
Why is this difficult? Brachos 4a asks why Yakov feared Esav in spite of Hashem's promise, and says that he was concerned for sin! Perhaps that is because he found himself in a dangerous situation, or he was worried lest he sinned through marrying two sisters, getting angry at Rachel and Lavan, benefiting from Lavan's money... (PF)
Why did Yakov stipulate about clothing? Hashem did not promise this!
Riva #1, Hadar Zekenim: Clothing to guard from heat and cold is part of Shmirah.
Riva #2: Hashem said "Lo E'ezavcha". One who has no clothing is abandoned - "v'Ohev Ger Lases Lo Lechem v'Simlah."
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 Yakov."
Ohr ha'Chayim: A vow must be spoken. If it is in the heart, it is not a vow. 1
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 2:29:31). 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)