The 1913 Catholic Encyclopedia says the following about St. Dymphna:
The earliest historical account of the veneration of St. Dymphna dates from the middle of the thirteenth century. Under Bishop Guy I of Cambrai (1238-47), Pierre, a canon of the church of Saint Aubert at Cambrai, wrote a "Vita" of the saint, from which we learn that she had been venerated for many years in a church at Gheel (province of Antwerp, Belgium), which was devoted to her. The author expressly states that he has drawn his biography from oral tradition.
Does the text of this Vita still exist today?