Rav Gidel said in the name of Rav: One who arises early and says, "I will study this chapter or this tractate" has made a great vow to his God. But is he not already sworn [to do so from Sinai] and a vow does not fall onto a vow?... We see that since he could have exempted himself by reciting the Shema in the morning and at night, therefore the vow fall onto him.What does it mean that he could have exempted himself by reciting the Shema in the morning and at night? The Ritva offers two answers: 1) He could have exempted himself from learning by, for example, having to work for a living. 2) He could have spent his time learning the passage of the Shema and not learning the passage about which he vowed.
Without looking at the later commentaries, it seems to me that there are a number of different ways to explain the point of divergence of these two approaches. One way is to say that the debate is whether the obligation to learn Torah is to constantly learn some part of Torah or it is to master all of the Torah. The second answer of the Ritva can be said to hold that one must simply learn Torah, regardless of what part of Torah. Therefore, one has not sworn to learn this particular chapter or tractate, because one can, at least theoretically, spend one's entire life studying the passage of the Shema and never move on to other areas of Torah. The first answer, however, holds that one is obligated to learn every minute part of Torah and, therefore, had to come on to the exemption of earning a living.
Now look at the Ran's comments on this matter. He is clearly of the view that the obligation is to learn every part of Torah and, therefore, uses a complex argument about whether the source of an obligation is explicit in the Bible or derived to explain the above passage.
On this subject, see this post.