This Question is pretty straightforward. Vos’s own commentary gets quite detail discussing this. I recommend getting a copy of Williamson’s update to Vos’ Commentary. Which is either paperback or hard copy. You can also access a PDF Vos Commentary here.
Notice the dual idea of the Bible’s usefulness for not only faith but the need to be obedient. There will be a lot of questions about our obedience later in the Catechism.
Q. 3. What is the Word of God?
A. The Holy Scriptures of the Old and New Testament are the Word of God, the only rule of faith and obedience.
Here is how the Study guide begins to discuss this “only” rule:
For what two things are the Scriptures our rule? The Scriptures are our rule for faith and obedience.
Why are the Scriptures our only rule of faith and obedience?
The Scriptures are our only rule of tilith and obedience because as the written Word of God they are unique and infallible, and therefore no other rule of faith and obedience may be placed alongside ofthent. This principle of course does not rule out such subordinate standards as the Larger Catechism itself, which present not another rule in addition to Scripture, but merely a systennatic summary of what Scripture teaches. The Larger Catechism, for example, is a legitimate rule of faith and obedience only because, and only so far as, it is faithful to the teachings of the Scriptures. It possesses no inherent authority of its own.
Rainbow Trout 
