Or HaChaim on Genesis 28:20/אור החיים על בראשית כ״ח:כ׳

1 וידור יעקב נדר לאמור. Jacob made a vow saying. The word לאמר is to remind us that unless a vow has been verbalised it is not valid. Mere good intentions do not rate as a vow.

2אם יהיה אלוקים עמדי, "If the Lord will be with me, etc." The word אם in this case is not to be understood as something conditional but is similar to the אם כפר יושת עליו in Exodus 21,30, or the אם in Exodus 22,24: אם כסף תלוה את עמי. In both those instances the meaning is "when," i.e. something that will definitely occur (compare Baba Kama 4,5 and Mechilta on Parshat Yitro)

3Jacob also included the attribute of Justice in his vow. So far G'd had spoken to him only in His capacity as Hashem, the attribute of Mercy. He undertook to tithe his acquisitions provided that also the attribute of Justice would prove to be with him. This is why he chose the expression אלוקים and not השם when formulating his vow.