Even saints are human. The fight between Vigilius and Boniface, while well known, did not stop either from being considered saints. Popes stood behind each when they were right and had Vigilius succeed Boniface after his martyrdom. Popes also rebuked them when wrong.
Fergal was a well traveled monk and an astronomer. Like many of his Irish compatriots, he set out to claim the world for Christ. He seems to have been a bit more accepting of things than Boniface liked but that did not hinder his ability to pastor and bring others to Christ.
Language and thought can be a tricky thing, especially at times and places when orthodoxy is prized for its ability to bring stability and souls to Christ but the language can be foreign to its hearers and speakers. So at times heterodoxy and diversity are just as important. Both of these saints embody them and were both successful in their own right.
If we have any complaint with Vigilius it is with his more human side. His zeal or perhaps laxity allowed for the forced conversion of Jews under his care, a fault towards politics that was pointed out to him by Gregory the Great.
Our faults do not keep us from becoming saints; it is our ridged adherence to those faults which damn us.
Theology and pastoral care go hand in hand. Theological doctrine that doesn’t let itself be directed and formed by its evangelizing purpose and by the Church’s pastoral concerns is no less unthinkable than pastoral activity that doesn’t know how to use Revelation and Tradition to better understand the Faith and preach it as Jesus commands.
-- Pope Francis, Oct. 27, 2016 Address to John Paul II Institute for Studies on Marriage and Family