Why did Hashem refer to the Neder that Yaakov made in Beis-El on his way to Charan?
Rashi, Ramban and Rashbam: He was reminding Yaakov that, having made the Neder - to turn the Matzevah that he set up there into a House of G-d (to sacrifice on it), he was now obligated to fulfill it. 1
The Ramban refers to all the parts of the Neder that Yaakov made in Beis-El (See above 28:20, 21).
Why did Hashem say "Asher Mashachta"?
Rashbam: You were Makdish it, and you did not offer on it yet.
It said that an angel was speaking to Yaakov. Why did he say "I am ha'Kel..."?
Tosfos ha'Shalem (2) #1: He should have said, "so said Hashem."
Moshav Zekenim, Tosfos ha'Shalem (2) #2: Hashem created an image [that Nevi'im saw when He spoke to them]. It is called an angel.
Why did Hashem call himself "ha'Kel Beis Kel"?
Malbim: There I promised to guard you and return you, and you vowed - now is the time to fulfill!
QUESTIONS ON RASHI
Rashi writes: "Ha'Kel Beis -El' - The letter Hey is extra." Why is this so?
Gur Aryeh: The word Kel is in construct form, "G-d of Beis-El," to which (in Lashon Kodesh) the definite article "the" is not usually added. 1
This is because "the" refers to a known entity which needs no elaboration. When a phrase is in construct form (A of B), it is being explained or clarified. (EK)
Rashi writes: "Mashachta - an expression of appointment to greatness." Why doesn't Rashi explain this simply as, 'you anointed with oil'?
Gur Aryeh: Anointment is relevant only for a Korban Mincha (Vayikra 2:4) [or on a person - in which the oil becomes absorbed]. Here we are speaking about a rock; so Rashi explains 'Mashachta' as appointment.
Rashi writes: "Where you made a vow - and you must fulfill it." Why does Rashi add this?
Gur Aryeh: Otherwise, this phrase would be extra.