"This loaf is forbidden to me" to the Ran and Rambam is the main neder vow. So why is he says, "This loaf is forbidden to me like meat that was not slaughtered properly [nevala]," is permitted? Because when one says, "This is forbidden to me like such and such a thing" the "such and such a thing" has to be something that can be forbidden by a neder [vow] or like someone who says, "This loaf is a present for the temple" [things presented to the temple are forbidden in use.] [This is a special law. For you might ask what is the difference  between "This is forbidden to me" and "This is forbidden to me like nevala"?] [This is not like the other Rishonim that hold the real neder vow is when one says "This is forbidden like a sacrifice to the temple" and the only reason "This is forbidden to me" works is as a extension [yad]]

However the very well known question on the Ran at the very beginning of tractate Nederim is that at first glance he seems to contradict himself. At first going like the Rambam and then going like Tosphot on the very same page.

Rav Shach says that for the actual law of neder is without attaching the prohibition to anything else [as the Rambam says] but for the language to make clear what he means [as is necessary for nederim] he has to say 'like a sacrifice. 

This to me seems like a very good answer to show that the Ran does not contradict himself. However the remaining question is that the actual language of the Ran does not seem to accept this explanation. 

What I means is that the Ran [Rabbainu Nissim] says in tractate shavut the reason you need "like a sacrifice" is because הקדש עושה חליפין if one says this animal is like that sacrifice, that is valid. The second animal becomes a sacrifice also. 

This me this seems like a contradiction to the idea of Rav Shach [in the beginning of Laws of Vows] 

[However a further point is that if the main neder is ''This is forbidden'' then it is hard to see that the very words ''this is forbidden'' would be thought not to count as a neder--and only valid as a short way of saying ''this is forbidden as a sacrifice'' when ''this is forbidden as a sacrifice'' is only forbidden because it is thought of as an extension of the main concept of neder. ]

 However one can answer this thus: in the way that Rav Shach explains the Rambam. I.e., that This is forbidden as a karban sacrifice is not valid as an extension of This is forbidden. Rather it is its own separate law. So This is forbidden is the main prohibition but still the language has to serve as  a meaningful way of saying you are forbidding something to yourself or to another, Not as saying that is is already forbidden before you make that statement. And that would in fact be meaningless.