The person making the vow could be from any class in society, though some might have more trouble fulfilling it than others, especially when the pilgrimage in question was a long one. Married women usually had responsibilities at home and usually had to get the permission of their husbands. Nuns needed permission from their abbess and sometimes the bishop. Priests had responsibilities to their parishes, while many nobles needed the permission of their king.
A vow of pilgrimage could be made privately to oneself, in the presence of a few witnesses, or publicly. Some were made with the serious intention of going, but it was not unknown for people to make these vows more frivolously, without thinking things through, and then find themselves bound by them. Women especially were warned against it, partially due to the common belief that they were less likely to really think things through and partially because it was simply harder for women to get away to go on pilgrimage. When people did find themselves unable to keep their vows, they would have to apply to a Church authority to be freed from them.
It was almost impossible to prove that someone had made a private vow. If making the pilgrimage turned out to be impossible and one’s conscience didn’t see it as a problem, nobody ever had to know. On the other hand, it was also impossible to prove someone hadn’t made a vow. So it was probably equally easy to claim the vow had been made and, provided there wasn’t too much to do at home, take a day off by going to visit some nearby shrine. A vow, after all, had to be fulfilled.
Webb, Diana. Medieval European Pilgrimage. New York: Palgrave, 2002.
Webb, Diana. Pilgrims and Pilgrimage in the Medieval West. London: I.B.Tauris Publishers, 2001.