Why does the Torah repeat the word "Ish"?
Rashi (on Pasuk 25): To include Nochrim in the law of Nedarim and Nedavos. 1
Menachos, 73b: To bring exclusively Olos - as the Pasuk concludes "le'Olah".
What is the difference between Nedarim and Nedavos?
Rashi (from Kinim, 1:1): Neder is when a person says 'Harei Alai'; Nedavah, when he says 'Harei Zeh'. 1
Ramban: Which he designates and brings immediately. Consequently, he is responsible for the former (to replace it should it die or gets stolen), but not for the latter.
Why does the Torah add "l'Chol Nidreihem ul'Chol Nidvosam"?
Ramban: To teach us that even a Nedavah, which one donates when not under duress, 1 must be without a blemish.
Seforno: To disillusion those who think that, since they volunteered to bring the Korban Olah, they are entitled to bring a Ba'al-Mum should they so wish. 2
What is the connection between the previous Parshah and the forthcoming one?
Seforno: The previous Parshah discussed the laws of those bringing the Korbanos; the forthcoming Parshah, the equivalent laws concerning the Korbanos themselves.
Rashi (25) writes that "Ish Ish" includes Nochrim in the law of Nedarim and Nedavos. We should know this already, for it says about Mumim "umi'Yad Ben Nechar Lo Sakrivu", i.e. but we may offer Tam animals for them!
Riva (25) #1: If not for "Ish Ish", one might have thought that one who offers a Tam for a Nochri transgresses an Aseh, and for a Ba'al Mum there is also a Lav.
Riva (25) #2: If not for "Ish Ish", one might have thought that Nochrim may offer even Chata'os and Ashamos. The verse teaches that a Tam for a Nochri transgresses an
Moshav Zekenim citing Ri: Nochrim may offer a Ba'al Mum on a Bamah. "Umi'Yad Ben Nechar" is not extra; it is needed to teach that they cannot offer it in the Mikdash. 1
Even if it is not extra, we can infer that they can offer Nedarim and Nedavos! (PF)
Why does it say "Lefalei Neder"?
Ramban, Moshav Zekenim: A Neder is a vow that one makes when things are Nifla'im (hidden) from them, and they promise to bring an Olah or Shelamim if He performs with them a Pele (miracle) and delivers them from their troubles. We find "Lehafli Neder" also below 27:2, Bamidbar 6:2). See also Bereishis 28:20 and Bamidbar 21:2.