Why did Yaakov refer here to Hashem in the singular, whereas in 35:7, the Pasuk refers to Him in the plural?
Sanhedrin 38b: To counter the heretics, who infer from 35:7 that there are multiple gods. 1
How does this refute them? They 'derive' from "Niglu Elav Elokim" (35:7) that there are many gods. They can say that here, Yaakov made a Mizbe'ach for Kel (the one of them) who answered him! (PF)
What exactly, is Yaakov saying in the latter half of the Pasuk?
Oznayim la'Torah: With reference to the Neder that he made on his outward journey to Charan, he is saying that, since the condition that Hashem would look after him had been fulfilled, he was now obligated to fulfill his part of the Neder.
Malbim: Chovos ha'Levavos (3:6) says that normally, one who thanks Hashem also intends to request for the future (that Hashem should continue to do so for him). Yaakov specified that the Mizbe'ach is purely for what Hashem did for him.
Ha'amek Davar: He explained [to his household] that he is not going to fulfill his vow and thank Hashem for the past. Rather, it is a prayer, that just like Hashem did for me in my affliction, so he will answer me now in my affliction. Hashem did not explicitly say so; Yaakov inferred it, for Hashem told him to build a Mizbe'ach, and not the Matzevah that he had vowed. 1 It says "ha'Oneh" in the present - just like He answered, He will answer me now.
What did Yaakov mean by, "... to the G-d Who answers me on my day of distress"?
Maharal (Nesivos Olam, Nesiv ha'Teshuvah Ch. 2, p. 152): This is Yaakov's trait - to vow [unto Hashem] at times of trouble. Indeed, we learn of Teshuvah in times of trouble from Yaakov Avinu.
