Q. If a neder works by forbidding an item onto oneself; why does the world say "Bli neder" to avoid some kind of commitment? wouldn't it be more appropriate to say bli shavoua?
A. The basic difference between a neder and a shevua is that a neder is a prohibition that takes effect on the item vowed upon, while a shevua takes effect on the person. (See Rambam H. Nedarim 1: 1).
Therefore, indeed the commitment accepted for doing something in the future, is a shevua and not a neder.
Horav Shlomo Miller Shlit'a explained that when uttering a promise to do a mitzva it is different and it becomes a neder, as when donating to tzedaka or to offer a korban,
Horav Dovid Pam Shlit'a added, that it may also refer to the Talmud (Nedarim 8a). It teaches in the name of Rav Gidal in the name of Rav, that if one says "I will get up early and learn this perek or this mesechta," he has pronounced a great neder to the G-d of Israel.
Ran (ibid.) explains that this is indeed a shevua, but often the Talmud uses the term neder to connote a shevua.
See also Tosafos (ibid.) for other interpretations.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit'a
Important Numbers
