Genèse 28:21

and will bring me back in peace to my father's house; the Lord shall be my God:

THEN THE ETERNAL SHALL BE MY G-D. This is not a condition, as Rashi would have it. It is rather a vow, and its purport is as follows: “If I will return to my father’s house, I will worship the proper Name of the Eternal in the Chosen Land at the location of this stone which will be for me a house of G-d, and there I will set aside the tithe.” There is in this matter a secret relating to that which the Rabbis have said: “He who dwells outside the Land of Israel is like one who has no G-d.” [Thus, according to the meaning of the above quotation, the Eternal will be Jacob’s G-d only when he returns to the Land of Israel.]

ושבתי בשלום, in accordance with G’d’s promise in verse 15.

והיה ה' לי לאלוקים, to assist me in all my undertakings.

ושבתי בשלום, from all the problems besetting man, i.e. a reference to what the Talmud had called רוח רעה.

והיה ה' לי לאלוקים, then G’d will be a judge, and He can examine to see if I did not fulfill my vow to the best of my ability. The letter ו at the beginning of the word והיה, [where we would perhaps have expected Yaakov to use an ordinary future tense such as יהיה, Ed.], may be in lieu of the word הנה, meaning that Yaakov would already from now on act on the assumption that G’d would provide him with the necessities he had asked for and therefore he was willing to be judged by the attribute of Justice, not relying on the attribute of Mercy.

ושבתי בשלום, with his wife and children, for without them neither he, his father, or his mother would consider themselves at peace, seeing that he was forbidden to marry one of the Canaanite girls.

והיה ה' לי לאלוקים, this was the gist of the vow, meaning that he would concentrate on the service of the Lord and minimise any mundane activities. Furthermore, he vowed that he would build an altar on the place where he had slept on the stone which he had just anointed.
יהיה בית אלוקים, he would build a Temple and altar there so that anyone worshipping the Lord could come there and pray and offer his sacrifices on that altar. It would be a Temple for the G’d in heaven only. Another way of understanding these words could be that he would instruct his children to build a Temple there in the future when they would take possession of the land of Israel.
A further vow Yaakov made at this time was: וכל אשר תתן לי עשר אעשרנו לך, I will give a tithe of all the acquisitions including the cattle which You will give me. I will distribute this tithe to needy people in order to encourage them to switch to monotheism. According to Bereshit Rabbah 70,7 the vow included tithing one of his future children to G’d, i.e. appointing him as a servant of the Lord representing the other sons. [As an interesting aside, the Midrash there claims that assigning only one of 12, respectively 14 sons, if we add Ephrayim and Menashe, Yaakov did not “tithe” i.e. give 10% of his children to the Lord, he is reported as having replied that seeing that each of his four wives had one firstborn, someone whose duty it was as a firstborn to devote himself to spiritual matters without his father “donating” him to G’d for such a task, he Yaakov had done well to additionally assign Levi as a “tithe” for the remaining 10 “children.” Ed.] As to the formulation: