Also vagus priests are against Canon law, for every priest must have a superior. Fr. Hewko, Fr. Pfeiffer, Fr. Roberts, Fr. Raphael, etc, do not have a superior.
Furthermore, run, don't walk, from any priest who believes himself to be the last vestige of the Catholic Church.
I would add that PERHAPS, in a different universe, it might theoretically be justifiable for a priest to "go it alone" in a time of Crisis. But that is not the kind of Crisis we are in today.
As of 2021, there are (4) bishops in the Resistance, holding the classic SSPX position. There are also (3) bishops in the SSPX, if you're big into Church authority and making peace with the modernists. And for those of a Sede persuasion, there are plenty of such bishops around.
My point: For EVERY PRIEST, regardless of his position on the Crisis, there IS a good bishop he can trust and PLACE HIMSELF UNDER, IN OBEDIENCE. There is NO EXCUSE. Any excuse he might give is a lame excuse, which basically says, "I like to be my own boss. I prefer to follow my own will than that of others." Don't we all!!! But that's going with Original Sin, and following the Broad Path -- not the narrow one which leads to heaven.
God has provided good bishops. Maybe there weren't enough faithful bishops to go around in 1975. But in 2021, especially with the Internet, there's simply no excuse.
I'm sure the devil has tempted many ex-Resistance priests to "go it alone" and have their own cozy, comfortable little apostolate, going into business for themselves. But the devil is laughing, because now the bishops can't organize anything big-picture, because they don't have enough priests to cover their chapels -- so all kinds of good isn't getting done.
The devil is managing to side-line young, healthy, valid, well-trained priests in little chapels of 50 or less people, so that 100x that number will not receive graces and benefits as they WOULD if that priest placed himself at the disposal of a good Bishop. A lot of these priests have little more than a lame excuse. "I don't want to travel." or "I don't want to follow another's will."
Guess what? A priest is fundamentally a bishop's helper. We're talking on the most fundamental level. That's why a priest isn't sufficient to himself. Notice a priest can't consecrate Holy Oils, give Confirmation, or ordain priests? That's because the Bishop is the self-sufficient one. A priest is fundamentally supposed to help a given Bishop -- the Bishop was the one with a ring, married to his diocese.