1st of all, one has to see that this "love letter", between the "Beloved"-a type or figure of Solomon, but not Solomon himself, and the Shulamite, as a 'type' of the nation of Israel, of whom she is a 'daughter'. There are numerous textures and layers regarding the Song of Solomon; at it's basic level it's a "song" describing the intimacy between a soul and her 'beloved', or the Lord.
It's most common interpretation is that it describes Israel's relationship with God; this is how the Mishneh rabbi's interpreted it from Rabbi Akiba:
R.Akiba said: I will speak of the beauty and praise of God before all
the nations. They ask Israel and say, 'What is your beloved more than
another beloved that "thou dost so charge us' (Cant. V, 9), 'that you
die for Him, and that you are slain for Him' as it says, 'Therefore
till death do they love Thee' (a pun on Cant. I, 3), and 'For thy sake
are we slain all the day' (Ps. XLIV, 22). 'Behold,' they say, 'You are
beautiful, you are mighty, come and mingle with us.' But the
Israelites reply, 'Do you know Him: We will tell you a portion of His
renown; my beloved is white and ruddy; the chiefest among ten
thousand' (Cant. V, 10). When they hear Israel praise Him thus, they
say to the Israelites, 'We will go with you,' as it is said, 'Whither
has your beloved turned him that we may seek him with you?' (Cant. VI,
1). But the Israelites say, 'You have no part or lot in Him,' as it is
said, 'My beloved is mine, and I am His' (Cant. II, 16)
The Song of Solomon is read during the Passover, so not only is it representative of Israel's relationship with God, but also an individual's relationship to God. "My mother's sons"(vs. 1:6) would only make sense if the father were different then her father. Both "mother's sons and the Shulamite" have vineyards, so it is apparent that "fields where grapes grow" isn't the context here, but Israel's search for an identity. Rashi is quoted:
my mother’s sons were incensed against me: These are the Egyptians
among whom I grew up, and they went up with me in the mixed multitude;
they were incensed against me with their enticement and their
seduction until they made me
…
a keeper of the vineyards: and there the sun tanned me and I became
blackened; i.e., they made me a worshipper of alien gods, but my own
vineyard, which I had from my forefathers, I did not keep. We find
that in the Scripture, leaders are called by an expression of
vineyards, as it is said (Hos. 2:17): “And I will give to her her
vineyards from there,” which the Targum renders: “And I will appoint
her leaders for her.” And similarly (Job 24:18): “he will not turn by
the way of the vineyards.”
We can infer that Israel was 'a vineyard' which the Lord chose out of all the nations, and redeemed from Egypt; to draw her to himself and give her an identity.
In a Christian context, vineyard is synonymous with 'life', or the human heart. C.H. Spurgeon said, in a message entitled, " The Unkempt Vineyard, Or Personal Work Neglected",
I invite all my fellow-servants to take a retrospect, and just to see
whether they have kept their own vineyards. I suppose that they have
worked hard. I only put the question Have they kept their own
vineyards? Have they served the Lord in all things?
Again, Rashi applies the same comparison in vs. 1:14,
in the vineyards of Ein-Gedi: The name of a place, and there it is
common. I saw in an aggadah (Song Zuta) that those vineyards produce
fruits four or five times a year, and this is symbolic of the many
atonements and forgivenesses that the Holy One, blessed be He, forgave
them for the many trials that they tried Him in the desert."
The next usage of vineyard(vs. 2:15) is interesting because in the Hebrew version that the Rashi commentary is in, it says,"
Seize for us the foxes, the little foxes, who destroy the
vineyards, for our vineyards are with tiny grapes
His commentary on this passage is,"
Seize for us the foxes: The Holy One, blessed be He, heard their
voice, commanded the sea, and it inundated them. That is the meaning
of “Seize for us” these “foxes,” the little ones with the big ones,
for even the little ones were destroying the vineyards, when our
vineyards were still with סְמָדַר, when the grapes were tiny. When a
Jewish woman gave birth to a male and hid him, the Egyptians entered
their houses and searched for the males, but the baby was concealed,
and he was a year or two old. So they would bring a baby from an
Egyptian home; the Egyptian baby would speak, and the Jewish baby
would answer him from his hiding place; and they would seize him and
cast him into the Nile. Now why does he call them foxes? Just as the
fox looks to turn around to flee, so did the Egyptians look behind
them, as it is written (Exod. 14:25): “I shall flee from before
Israel.”
In this case, the NASB version seems to 'separate' vineyard and 'vineyards'.
Perhaps the only instance where one could make the case for a 'literal' vineyard is vss. 8:11-12. Rashi stays definitively with his interpretation,
Solomon had a vineyard: This is the congregation of Israel, as it is
said (Isa. 5: 7): “For the vineyard of the Lord of Hosts is the House
of Israel.” In Baal-Hamon: in Jerusalem, which is populous and has a
multitude of people. Baal: an expression denoting a plain, like (Jos.
12:7): “from Baal-Gad in the valley of Lebanon.” he gave the vineyard
to the keepers: He delivered it into the hands of harsh masters:
Babylon, Media, Greece, and Edom. In Midrash Shir Hashirim, I found
some support to [the theory that] these keepers are the kingdoms.
each one brought for the fruit thereof: whatever they could collect
from them: head taxes, tithes, and illegal foreclosures; they
collected everything from them to bring into their homes.
So, to answer the question, "Israel, in her relationship with God" is the best way to understand "vineyard". Isa. 5:7 says,
"For the vineyard of the LORD of hosts is the house of Israel."
However, it is my contention, along with many in Christianity, that Solomon went 'beyond' the metaphor of a love relationship between Israel and God, and described an intimacy paralleling sexual union between a 'spouse'(soul) and her beloved-the Lord.
