Dear Kollel Iyun ha'Daf,
How does the Gemara (just before the end of the Perek) bring a proof that Rebbi Tarfon's opinion is that Asmachta Lo Kanya from Nazir? Perhaps that which Rebbi Tarfon requires clarity inherent in the acceptance of the Neziros is a Din specific to Nezirus, as the Gemara seems to clearly state: "Lefi she'Ein Nezirus Ela l'Hafla'ah"?
Moshe Binyamin Cohen, Yerushalayim, E"Y
The Rivan in Tosfos in Sanhedrin (25a DH 'Hahi') asks your Kashya. They give two answers ...
1. ... even though Nezirus is derived from a Pasuk, we can still learn other things from it (needless to say, this answer begs further explanation).
2. ... in the name of the Ri that since the Pasuk does not define Hafla'ah, we need to do this ourselves from a S'vara, and it is R. Yehudah Amar R. Tarfon who supplies the definition.
Once we know that, it is logical to say that the same will apply to Asmachta (that stems from a lack of knowledge). If such an Asmachta would be Koneh, then there would no reason for a similar case (where there is a lack of knowledge) not to be considered Hafla'ah. And the reason for this is because it would then fall under the category of Yados, which take effect even though part of the Neder was not specified.
Conversely therefore, since according to R. Yehudah Amar R. Tarfon, a lack of knowledge is not considered Hafla'ah, he must also hold that Asmachta is not Koneh. Both cases in turn, are based on the S'vara that when the person who makes the declaration is uncertain whether he is Chayav or not, he does not obligate himself to pay or to carry out what he has undertaken, whatever he has undertaken, as the Gemara concludes in Eruvin, and it is unclear why Tosfos omit it.
be'Virchas Kol Tuv
Eliezer Chrysler.