How can a neder to be a nazir for 500 years be a proper neder any more than a neder not to sleep for 3 days=no one lives to be 500 years old?
It seems to me that the difference between them is that one is clearly an exaggeration (500 years) and one is not clearly an exaggeration, but rather someone who makes an oath to stay awake slightly more than the amount that Chazal said is possible. Accordingly, the intent of staying up for three days is looked upon as real, and being that it is known to Chazal that this is impossible he receives lashes. However, the intent of the former is clearly looked upon as wanting to be a Nazir for as long as he lives, not that he, nor anyone, thinks he will live for five hundred years.
All the best,
Yaakov Montrose
1. The reason he gets lashes for swearing not to sleep for 3 days is shavuos shav, not that he violated his shavuas bitui. He is not bound by a shavuas bitui at all because it is not possible to do it. If he makes a neder not to sleep 3 days, he can sleep right away and will not get lashes, because there is no such thing as a neder shav. If he is not required to lose any sleep, why he is required not to drink wine? Why don't we treat his neder/shavua as a commitment to not sleep until he drops?
2. The answer seems to imply that a shavua not to sleep for 500 years would be binding just like the nazir for 500 years, that there is a difference between 3 days and 500 years. Is there a source for this?
Both questions can be answered by asking ourselves one question: Although his intent is unclear from the meaning of his words alone, is his intent clear from the context of his words? When someone says "I am going to be a Nazir," those words alone bind him to Nezirus. Even if he does not proceed to qualify his statement, we know that Stam Nezirus Sheloshim Yom. The qualification of those words with a description of how long he is going to be a Nazir is something that is used to determine the ultimate length of the Nezirus.
However, the words "I am not going to sleep" do not give a person a status of not having to sleep. The entire statement is necessary to determine exactly what he is stating. Stating that he will not sleep for three days in the form of a Neder means nothing (as you noted above). Accordingly, the Nazir's statement is looked at as qualifying his Nezirus in an exaggerating manner to mean as long as possible, while the entire statement regarding not sleeping is looked upon as just hot air.
All the best,
Yaakov Montrose
I would like to add a few words of explanation to complement Rabbi Montrose's answer. In the case when someone swears that he will not sleep three days, it is the act that the person has accepted upon himself (or the lack of action, in this case) which is not in his control. However, in the case of the Nazir, the person has accepted to simply keep the laws of Nezirus. Living 500 years is not part of what he accepted upon himself to do or not to do in his oath. It is simply a qualification of how long the Nezirus should last. Therefore it cannot invalidate the Nezirus.
Best wishes,
Mordecai Kornfeld