The Reshash asks that since one is not obligated to learn that specific tractate, the vow should be binding to learn that specific tractate? Actually, the Ritva uses this approach to understand what the Gemora is saying. Since one can fulfill their obligation with some other type of learning i.e. kerias shema, or any other tractate, therefore, when he makes a vow on a specific tractate, it is fully binding.
Tosfos writes that even if one makes a vow not to learn something specific, the vow is binding. Reb Avi Lebowitz cites Reb Moshe (Dibros Moshe heora #43), who explains that Tosfos cannot be understood to be saying that one is not obligated to learn all sections of Torah, because both the obligation of knowing Torah and the obligation of constantly learning Torah actually requires a person to learn all sections of Torah every day. While it may be impossible to do so, there is still technically an obligation on every single aspect of Torah. Therefore, Tosfos can only be explained like the Ran that the obligation to learn every section of Torah is not stated explicitly. Based on this, the Rosh and Ramban would hold that a vow not to learn even a specific or obscure section of Torah on any particular day would not be binding at all.