A person who takes a neder to keep nezirus for one day, for example, is automatically a nazir for 30 days, as that is the minimim shiur of nezirus. Similarly, if a person takes a neder to be a nazir only from drinking wine, there is no such thing, and he becomes a full nazir with all the associated prohibitions. Minchas Chinuch has a safeik whether there would be bal yacheil in these cases if the nazir violates the part of nezirus that he has not verbally accepted, but which halacha imposes upon him. Is the issur about contradicting the words that come out of a person's mouth, or is the issur about undermining the effect/outcome of those words.
The Rambam paskens that if Reuvain takes a neder that all of his property should be assur to Shimon and then gives Shimon something he owns, Reuvain would get malkos for violating bal yacheil. The Ran, however, disagrees and holds the opposite: it is Shimon that gets malkos for violating the neder, not Reuvain (see Lechem Mishna, Hil Nezirus 5:1). The Steipler points out that here we have a test case where Reuvain is the one uttering the words that are being violated, but the effect those words have stop Shimon, not Reuvain himself, from doing something. Rambam is mechayeiv Reuvain in malkos because Rambam understands bal yacheil as a violation of the words. Ran is mechayeiv Shimon because he looks at the net effect.
One can also understand the safeik of the Minchas Chinuch is a more narrow way. Accepting upon onself a neder of nezirus is not really the same as taking a neder not to drink wine, take a haircut, and become tamei. Achronim point out (see Avnei Miluim in the teshuvos #15 quoting Mahari"t, also brought by the Shalmei Nedarim on 2b in Nedarim) that the issurei nezirus are the result of a chalos that transforms the person. A person who says "hareini nazir" never mentions wine, haircuts, or tumah, but nontheless becomes a nazir and as a result is bound by all those issurim + bal yacheil for violating them. So perhaps the safeik of the M.C.. applies specifically to nezirus, because specifically here bal yacheil is about violating a status, not about violating specific terms/words of a neder.