Tractate Nedarim is similar to tractates Nazir, Shevuot, and to a degree, Arakhin, in that they all address obligations that are not imposed by the Torah but are voluntarily created by an individual. In a broad sense, the category of vows includes nazirite vows, oaths, and consecrations, which are the primary topics addressed in those tractates.
These tractates are connected not only with regard to their general topic but also with regard to the structure of the tractates and the halakhic details that are common to these different manifestations of vows.
Tractate Nedarim, which addresses the topic of vows in a more narrow sense, is found in the order of Nashim because most of the verses in the Torah that address this topic refer to vows of women. However, the halakhot apply equally to vows made by anyone.