"In that day - 'Sing about a fruitful vineyard: I, the Lord, watch over it; I water it continually. I guard it day and night so that no one may harm it.'" NIV translation
The "vineyard" speaks of God's people and this "fruitful" one is a stark contrast to the vineyard that was described in chapter 5. There, God was pushed out of the lives of His people and was treated as just a religious obligation but, here, they are in a close relationship with God.
This passage is close to John's description of the New Jerusalem as he described the river of life flowing from the throne of God (see Revelation 22). Unlike the previous city that trusted in its walls and gates to protect it, the gates of this city will always be open as it is directly protected by God. John described this in Revelation 21.
"'I am not angry. If only there were briers and thorns confronting me! I would march against them in battle; I would set them all on fire. Or else let them come to me for refuge; let them make peace with me, yes, let them make peace with me.'" NIV translation
Isaiah now returns to talking about the people of his time and the fact that they were not fully trusting in God. Although God would be just in being angry, He issues this plea for Israel to accept His peace. They had not totally abandoned their relationship with Him but it was not an exclusive relationship as they also allowed idols.
This is very similar to the "lukewarm" description that Jesus uses in His letter to the church in Laodicea (see Revelation 3:15). The "briers and thorns" speaks of weeds or those that reject the offer of salvation. Those that do so are removed and judged but there is also a judgment ahead for those that are only lukewarm. God is pleading with His people to forsake all other things and to trust in Him alone. There is only one way to become a resident of this shining city and that is through total faith in the finished work of Jesus Christ.