In the case of one who vows "I will become a nazir at some point in my life." This seems like a license for someone not to fulfill their vow at all! Obviously, one cannot know when they will die. So if you die before you fulfill your neder (or if you die in the process of fulfilling before the thirty days are up) are you chayav?
B. Rosenzwig, Missoula, MT, U.S.A.
There are different explanations of this Gemara.
The Rosh quotes an opinion which states that because a person must fulfill their vow right away in case they die, they must become a Nazir within three Regalim from making the Neder or they transgress Bal t'Acher.
However, the Ran indeed seems to say that if this person does not become a Nazir right away he transgresses Bal t'Acher, as he is leaving himself open to transgressing Bal t'Acher.
Moral of the story: Don't write spiritual checks that you can't cover.
Take Care,
Yaakov Montrose