The gemara says that if one made a nezirus on the condition that they should be permitted to drink wine it would be a condition against the torah, so the condition will be void and the statement of "i should be a nazir" is binding. Tosafos asks, why does the gemara invalidate the condition based on the concept of making a condition against the torah, the condition should be invalid for another reason, namely, if one can't make a messenger to do it he cannot make a condition on it (tosafos answers that since the korbanos can be done by others, even though other aspects of the nezirus cannot be done by other, it qualifies as able to be done by others to permit making a condition about it). R' Akiva Eiger asks why does tosafos feel that the rationale of not being able to make a messenger is a better reason to invalidate the condition than the fact that it is a neder agains the torah. The question is predicated on the assumption that Tosafos is simply asking that instead of saying the rationale of a condition against the Torah, we should say the rationale of not possible through a shaliach, and therefore asks why is one any better than the other. However, it seems to me that tosafos question is more significant. If the concept of not able to be done by a shaliach applies here to invalidate the condition then the ramifications would be more far reaching and invalidate any condition placed on nezirus. Tosafos is asking, why does the gemara only consider it to be a problem of a condition against the Torah which would be limited to the specific case addressed in the mishna; the gemara should say the problem is that it can't be done through a messenger so that one can never make a nezirus on condition.
Tosafos takes for granted that nezirus cannot be done through a shaliach. The maharatz chiyus has somewhat of a discussion about this and cites a noda b'yehuda who says simply from the fact that tenai works by neder, it must be possible through a shaliach. Perhaps Tosafos makes a distinction between neder and nezirus, and holds that you can do a neder through a shalicach like the noda b'yehuda which would answer R' Akiva Eiger second question. BUT that distinction itself would need to be explained.