If the hair is willingly given, then there's no reason why a vegan shouldn't wear a wig made of human hair.
Vegans are vegans to minimise their contribution to animal suffering and exploitation. Where no exploitation is involved, there can be no objection.
BUT concern about ethically-sourced human hair is not confined to vegans or vegetarians. Many people, whatever their dietary choices, will want to make sure that the hair is not from, for example, prisoners in some countries who are forcibly shaved so their hair can be sold, or people living in grinding poverty in Asia who have sold their hair for a pittance so that some charlatan can make a huge profit from it – and yes, these things do happen, and hair for wigs is sourced in that way. Vegan or not, that should be a consideratioin when looking for a wig
Nobody can have any objection to an ethically-sourced human hair wig, for example one from thecancer charity Locks for Love where people have voluntarily donated their hair out of compassion, or where someone has voluntarily sold their hair for a decent price.
Interestingly though, most human hair for wigs comes from Asia, and much of it will not have been donated voluntarily and/or obtained without exploitation