It seems to me that such a statement does not take effect, and he is not a Nazir at all.

1) The source for this is Tosfos to Nazir 2b, DH Mahu, who writes that any vow made mistakenly is not a vow. Therefore, if he intended to say, "These fruits are Ma'aser," and said instead, "These fruits are Terumah," it is as if he said nothing.

2) Similarly, the person who said "Hareni Nezirah" must have been intending to say "Hareni Nazir." Since a Nazir and a Nezirah are clearly two different things, it is as if he said nothing.

3) See also Shevuos 26b where the Gemara says that if one intended to say wheat-bread but instead said barley-bread, he is exempt. The Rashba in Kidushin (50a, DH v'Asikna) writes that this is because we require "Piv v'Libo Shavin" -- his mouth and heart must be equal, but if what came out of his mouth is not the same as what is in his heart, the vow is meaningless. The Rashba writes that this person made a mistake and it is not considered as speech but is a "slip of the tongue."

Similarly, the man who said "Nezirah" made a slip of the tongue and therefore is not a Nazir either.

4) There is a very slight hint that "Hareni Nezirah" is meaningless from Tosfos to Nazir 10a, DH Parah, who writes that the man who said "Hareni Nezirah" said it with a Mapik Heh (with a dot in the Heh at the end of the word "Nezirah"). This implies that if he would have said it with a regular Heh (at the end of the word "Nezirah") this would have been invalid.

Reb Chaim, I have found the Yerushalmi to which the Gadol referred you. It is in chapter 2 of Yerushalmi Nazir, on the second Mishnah in the chapter, i.e. the Mishnah cited in the Bavli on 10a, about the cow saying "Hareini Nezirah."

a. The Yerushalmi asks whether a man can be "Matpis" his Nezirus with the same words as a woman. The Yerushalmi answers by saying "there they say a Nezirah is passing by." The Korban ha'Eidah expains that the question is about a man who said "Hareini Nezirah." The answer is that "there" -- i.e., in Bavel, which is termed "there" in the words of the Yerushalmi which was written in Eretz Yisrael -- if a Nazir is passing by they say that "a Nezirah is passing by." This shows that a Nazir is sometimes called a Nezirah and it follows that if a man said "Hareini Nezirah," this works and he thereby becomes a Nazir.

b. So according to the Yerushalmi, if a man says "Hareini Nezirah" he thereby does become a Nazir.

c. I found that the Sefer Be'er Moshe (by Rav Moshe Charif) on Maseches Nazir (published in Livorno in 5612, and available on Otzar ha'Chochmah) writes that he did not find that the Beis Yosef cites the Yerushalmi, but he does not know why ,because we have not found that the Bavli disagrees with the Yerushalmi. He writes that Tosfos (10a, DH Parah, which I cited earlier) is forced to say a difficult Peshat -- that he says "I am Nezirah because of this cow," while according to the Yerushalmi one may say that he said "Hareini Nezirah" in the standard way and even so the Nezirus applies.

d. We see from the Be'er Moshe that he understands that according to Tosfos, when a man says "Hareini Nezirah" (without a Mapik Heh), the Nezirus does not apply.

e. I would also like to suggest that possibly the Yerushalmi does disagree with the Bavli. My support for this is the Gemara in Nedarim 49a, which says -- about the rule that for vows one follows the way people speak -- that this means specifically people in one's locality. This does seem consistent with the Yerushalmi about Nezirah because the Yerushalmi proves from the fact that "there" in Bavel they call a Nazir a "Nezirah," which means that in Eretz Yisrael if one says Nezirah it also is effective for a man. Even though people do not speak this way in Eretz Yisrael, the fact that people speak that way in Bavel is sufficient to make the Neder apply.

Kol Tuv,

Dovid Bloom