וידר יעקב, נדר לאמר: אם הי אלהים עמדי, ושמרני בדרך הזה אשר אנכי הולך, ונתן לי לחם לאכל ובגד ללבש. ושבתי בשלום, אל בית אבי והיה הי לי לאלהים. והאבן הזאת אשר שמתי מצבה הי, בית אלהים וכל אשר תתן לי עשר אעשרנו לך.
And Jacob uttered a vow, saying, "If G-d will be with me, and He will guard me on this way upon which I am going, and He will give me bread to eat and a garment to wear. And if I return in peace to my father's house, and the L-rd will be my G-d; then this stone, which I have placed as a monument, shall be a house of G-d, and everything that You give me, I will surely tithe to You."
This is not the only time we see this sort of conditionality in a vow (Judges 11:30-31, I Samuel 1:11, 2 Samuel 15:8). What makes Jacob's conditional vow unique is that G-d already promised Jacob the conditions for which he asked (Genesis 28:15). So what's going on?
Perhaps Jacob is wary of the dream's validity. It might have been an actual prophecy, but it just as easily could have been a dream. This could have been an instance in which Jacob was simply hedging his bets (Zohar 1:150b). Let's assume that Jacob was not skeptical of the dream's veracity, and he actually considered it to be a bona fide prophecy. Jacob's conditionality doesn't make sense if G-d already promised these provisions to Jacob.
Some commentators, such as Rashi, assumed Jacob used the word אם (if) because was legitimately unsure as to whether G-d will fulfill His promise. Ramban believed the word אם is used because Jacob fears that he might sin, and thus forfeit what G-d had promised. Radak pointed out that Jacob only asked for necessities, not luxuries, which is the behavior of righteous people. Although it might not seem spiritual to ask for material provisions provided, even if it's the bare minimum to survive, we have to remember that it's difficult, if not impossible, to keep to G-d's ways if we cannot even have the most basic amenities provided. That is why Sforno commented that Jacob's supplication will help ensure that he can follow G-d's will to the fullest and not falter.
Since the word אם can also mean "when," it is feasible that Jacob was expressing his faith in G-d and simply declaring what he would do once he returned in one piece. This interpretation is implicit in the Midrash (Genesis Rabbah 70:6) because the Midrash discusses how Jacob's vow was meant to be an example for how future generations are to praise G-d. According to this interpretation, Jacob's vow was not one of conditionality, but of the utmost confidence in G-d. Jacob has found faith in G-d, and when I say "found faith in G-d," I don't mean that G-d will literally provide for everything, but that we can be thankful for what we have and have enough of a sense of equanimity to know that we can adapt to whichever difficulties come our way in the future. Rather than be a petty form of spiritual quid pro quo, Jacob was actually on the spiritual path that would help him come to terms with himself and transition from being Jacob to becoming Israel.