Or HaChaim on Genesis 28:21/אור החיים על בראשית כ״ח:כ״א

1 והיה ה׳ לי לאלוקים. "Then Hashem will be my Lord." This sounds, G'd forbid, as if until now Hashem had not been Jacob's G'd. Rashi explains that Jacob meant that G'd would not find any of his children as morally unfit. I do not know where such a thought is alluded to in Jacob's words. [actually Rashi quotes a Sifri in Parshat Va-Etchanan section 31. Ed.] Besides such a promise, i.e. that his children would be morally fit, is at best a conditional promise [seeing that the matter lies within the will-power of the children, Ed.]. When would the stone then be able to serve as a monument for this encounter Jacob experienced during his dream? The proper explanation of what Jacob said is that he was prepared to conduct himself in such a way that G'd would consent to associate His name with him even during his lifetime. He had previously done this with Abraham after the latter's death and with Isaac only after he had become blind and therefore beyond the reach of the evil urge (compare verse 13). The words לי לאלוקים are clear evidence of Jacob's intention. This was a tremendous undertaking by Jacob. You will find in Genesis 33,20 that G'd reminded him of that undertaking and that Jacob then fulfilled this vow by erecting a monument in G'd's honour.