If there is an outstanding warrant in VT, chances are high. I sggest you consult with an attorney there.
Any response given is not to be taken as legal advice or to create an attorney client relationship.
If it is a federal warrant I'd say 100 percent.
He will probably be entitled to a federal public defender once he makes his first appearance before a federal magistrate so he will not be alone and floundering in court. The warrant should give some indication what it is for.
You can try to find out more information by going to PACER - a federal public website. Put in your brother's name and indicate (where required) that it's a Vermont case. You might get some help that way.
The fact that it is a federal warrant makes it very very likely they will extradite him.
Seth Weinstein, Esq.
Practicing throughout Southern California
This reply should NOT be considered a legal opinion of your case / inquiry. At this time I do not have sufficient factual/legal documentation to give a complete answer to your question and there may be more to the issues you raised then I have set out in my brief reply.