By faith [the Christian] is a member of the covenant [of grace], and that faith has a wide outlook, a comprehensive character, which not only points to justification but also to all the benefits which are in Christ. Whereas the Lutheran tends to view faith one-sidedly–only it its connection with justification–for the Reformed Christian it is saving faith in all the magnitude of the word. According to the Lutheran, the Holy Spirit first generates faith in the sinner who temporarily still remains outside of union with Christ; then justification follows faith and only then, in turn, does the mystical union with the Mediator take place . . . The covenantal (or Reformed) outlook is the reverse. One is first united to Christ, the Mediator of the covenant, by a mystical union, which finds its conscious recognition by faith. By this union with Christ all that is in Christ is simultaneously given. Faith embraces all this too; it not only grasps justification, but lays hold of Christ as Prophet, Priest, and King, as his rich and full Messiah.
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Vos, Geerhardus. "Doctrine of the Covenant in Reformed Theology" in Redemptive-History and Biblical Interpretation edited by Richard B. Gaffin, Jr. p 256.
The point that Vos makes is correct but, just as we have to be careful to contextualize his comments about union as central dogma (as Jeff pointed out in his review of Fesko) so too we have to contextualize these comments. Vos wasn’t making the same distinction in the 1890s, when he gave this lecture, as some contemporary formulators of union are making today.