ציטירנדיקער פּירוש אויף ישעיהו 62:8

It is well to note how Moses phrased this legislation when he commenced with זה הדבר אשר צוה ה'. He continues with איש כי ידור נדר לה', instead of merely saying איש כי ידור נדר אליו, repeating the word לה' unnecessarily. Moses had to repeat that word to allude to the power of the vow which is greater, i.e. above that of the שבועה, oath, a word limited to “seven” as it is derived from the word שבעה, seven, and subject to its limitations. Seeing that the vow possesses such a superior power it applies even to mandatory commandments and when one pronounces such a vow the wording of which overrides such a commandment this is not as if one had not said anything at all as is the case when one pronounces an oath the wording of which overrides such a commandment. The whole Torah legislation, i.e. the מצות, are products of the regions within these seven celestial regions They represent the seven different types of voices (sounds) the Israelites heard during the giving of the Torah as we explained in our commentary on Exodus 20,1. [Compare our translation ibid. Ed.] This also accounts for the fact that the preposition used by the Torah for the word שבועה, oath, is invariably the preposition ב, such as אשר נשבעת להם בך, (Exodus 32,13); בי נשבעתי (Genesis 22,16) or ובשמו תשבע (Deut. 6,13). נשבע ה' בימינו (Isaiah 62,8) “The Lord swore by His right hand,” Seeing that we learned in Nazir 3 that if someone says ימין [i.e. “by my right”] this is a form of oath, the idea of “swearing an oath” is associated with the party declaring it is a partner of the concept of the seven, שבעה, celestial regions and what they represent.
The word נדר, vow, on the other hand, is always associated with the preposition ל, pointing at something above the object or party referred to by the one making the vow. We have Psalms 132,2 נדר לאביר יעקב, or here איש כי ידור נדר לה', or in Psalms 24,1: לה' הארץ ומלואה. When you appreciate what we have just pointed out you will understand the meaning of Sifri Mattot 153 that whereas when someone renders an oath he swears by the person of the king, when he vows a vow he swears by the life of the king. The essence of the king is his life, in the case of the Lord, His eternal life. The Sifri there points to Elisha saying to Elijah “by the life of the Lord and the life of your soul, I will not abandon you,” as an indication that the expression בחיי is the ultimate statement of this sort (Kings II 2,7).

shall eat it This refers back to “your grain.”

shall drink it This refers back to “your wine.”

ואספת דגנך AND THOU SHALT GATHER IN THY CORN means: and thou shalt gather it into thy house, and not thy enemies, just as it is said, (Isaiah 62:8—9) “[The Lord hath sworn … ], surely I will no more give thy corn [to be food for thine enemies], but they that have gathered it shall eat it”; and not as it is said, (Judges 6:3) “And so it was when Israel had sown that the Midianites came up [and destroyed the produce of the land]” (Sifrei Devarim 42:10).