One claim echoed in both comments was that the phrase, "sons of God" is only used in the OT to refer to angels. I don't think this is sustainable when we actually do our homework rather than saying something simply because other people said it.
There are three questionable references in Job. Job 1:6, 2:1, and 38:7. The reference in ch. 38 refers to a time when the earth was created, so I am willing to conceed that in the book of Job, the phrase "sons of God" most likely refers to angels (as is no other class of beings).
Yet, that doesn't conclusively prove that the phrase "sons of God" always means "angels." I could easily give the example of James and Paul's usage of "justified" to demonstrate that it is possible that two different authors may use two different meanings for the same word.
Since the reference to "sons of God" is used in Moses, we should look to Moses and his usage first to determine the meaning of the phrase "sons of God." Doing a search on Biblegateway, I was able to find two references: Deuteronomy 14:1 and Deuteronomy 32:8
1"You are the sons of the LORD your God. You shall not cut yourselves or make any baldness on your foreheads for the dead. 2For you are a people holy to the LORD your God, and the LORD has chosen you to be a people for his treasured possession, out of all the peoples who are on the face of the earth.Deuteronomy 32
8When the Most High gave to the nations their inheritance,Now, I will conceed that "God" in Deuteronomy 32:8 is footnoted saying that the Masoretic Text reads "Israel" so I'm willing to say that it is possible that it doesn't include the phrase "sons of God" but it is clear that Deuteronomy 14:1 refers to the people of Israel. It preceeds commands from God about preserving the purity of the nation and obedience in various forms. Which is exactly what Vos is claiming the children of Seth failed to do in marrying the daughters of Cain.
when he divided mankind,
he fixed the borders[a] of the peoples
according to the number of the sons of God.[b]
9But the LORD's portion is his people,
Jacob his allotted heritage.
I still am firmly convinced that the "sons of God" refer to the children of Seth and the "daughters of men" refer to the daughters of Cain. Their intermarriage was displeasing to God's eyes because it resulted in sin and disobedience, people rejecting their Lord and seeking evil. This brought about God's just wrath and punishment through the flood.