In his Inaugural Address at Princeton, Vos makes the case that Biblical Theology is an exegetical enterprise. It is inherently tied to the text of Scripture and arises from the text itself. He argues that Biblical Theology follows the progress of revelation, chronologically and organically, from Genesis to Revelation.
Along the way, Vos makes one of those earth-shattering, astounding, observations regarding God's supernatural, salvific acts of redemption in history: the Exodus, the cross, and Christ's resurrection are self-revelatory; better yet, self-interpreting.
"The first feature characteristic of supernatural revelation is its *historical progress*. God has not communicated to us the knowledge of the truth as it appears in the calm light of eternity to His own timeless vision. He has not given it in the form of abstract propositions logically correlated and systematized. The simple fact that it is the task of Systematic Theology to reproduce revealed truth in such form, shows that it does not possess this form from the beginning.
"The self-revelation of God is a work covering ages, proceeding in a sequence of revealing words and acts, appearing in a long perspective of time....Revelation is not an isolated act of God, existing without connection with all the other divine acts of supernatural character. It constitutes a part of that great process of the new creation through which the present universe as an organic whole shall be redeemed from the consequences of sin and restored to its ideal state, which it had originally in the intention of God.
Having made the argument that God's great acts of redemption are themselves revelation and belong to the work of the Bible expositor, Vos then summarizes his definition of Biblical Theology, a discipline, he argues that belongs to the realm of exegesis:
"Biblical Theology...is that part of Exegetical Theology which deals with the revelation of God in its historic continuity...Biblical Theology, rightly defined, is nothing else than the exhibition of the organic progress of supernatural revelation in its historic continuity and multiformity." - Geerhardus Vos, "The Idea of Biblical Theology as a Science and as a Theological Discipline"; Vos's Inaugural Address as Professor of Biblical Theology in Princeton Theological Seminary, delivered May 8, 1894 (http://www.biblicaltheology.org/ibt.pdf).
The great salvation acts of God in history, especially those of the Christ Event (Christ's incarnation, life, crucifixion, resurrection, ascension/exaltation), are interconnected with everything else in his supernatural revelation we call the Bible. No text in the Bible is isolated or arbitrary. No text exists for its own sake, but for the ongoing progression of the one story of the Bible, that of the Person and work of Jesus.