The idea of a "neder" [vow] is different from an oath [shavuah]. The difference is that a vow is derived from the way one would vow something to the Temple. That is you would say "This animal I am vowing to be a peace offering." Or you might say, "I am vowing this chair." In that case you would bring the chair to a representative of the Temple [Gizbar] and they would sell it and the money would be used for the Temple. The whole idea of a vow comes from that. That is you can say, "This loaf of bread is a 'Karban' [sacrifice] to me". Or even less. You might say, ''This loaf of bread is forbidden to me'' and not even mention a sacrifice. You can even have "yadot nedarim" {extensions of vows}. That is where one does not even use the right words, or some broken version of the word.
So when it comes to a "herem" [excommunication], the same kind of set of laws applies. That is, that interacting with an individual or a group can be made to be forbidden. For example, I might say to myself, "so and so is herem to me." So then all interaction with that person is forbidden. But then you get the issue of the herem that the Gra signed. [There were a few. The Gra signed the second one]. However in order to have the authority to make a decree of excommunication one needs to have gone through Shas Talmud at least once. Not anyone can make a valid excommunication on a group that makes it in fact forbidden for anyone to have any connection with that group. But seeing that the Gra in fact went through Shas at least once, it is safe to say that that excommunication is valid.
And in addition to this I ought to make the point that one is obligated to remove a stumbling block in front of people. אל תעמוד על דם רעך/ לפני עוור לא תיתן מכשול. So if one is aware of this problem and yet ignores it or pretends that it doesn't exist. that in itself transgressing. One does not get off scot-free by a plea of lason hara/ slander. It is not lashon hara to warn someone of a danger to their body and soul.
[By the way I do not think it applies to Rav Nahman of Breslov. You can see why yourself by looking up the actual language of the herem.]]
