Thank you for posting this. It does give fuller light to the discussion. I struggle with the rest of your logic. As you noted, Muller goes on to say -Muller's "may also have" qualifier hardly implies Vos actually suffered from the same issues as did Berkhof sans a prolegomenon, not to mention
I think this gets to the heart of the issue. When we study theology, an important component is Revelational epistemology, and the doctrine of scripture. These are discussed in a good prolegomenia. This is why I find Bavinck's RD very helpful (vol 1 in this case). A prolegomenia is missing in Vos. Further, with the rise of mysticism in certain sections of the church, a Revelational epistemology is more important than ever.As can easily be seen from Berkhof’s Introduction, moreover. Reformed theology assumes that a whole series of issues must be addressed before one comes to the doctrine of God. not the least of which is the identification of Scripture as the principium cognoscendi or "cognitive foundation." and God as the principium essendi or “essential foundation" of theology. As Berkhof makes clear. Christian theology cannot be based on “a priori speculation" and is. therefore, never to be systematic in the sense of a speculative or deductive philosophical system. Rather. Reformed theology rests on biblical revelation as its only cognitive foundation or principium in a way that is more inductive than deductive.
Please note: I am NOT denying the importance to the church of Vos' RD. Vos was a top rate theologian. It seems to me the idea is to read Bavinck's prolegomenia alongside Vos' RD.