Where are his other exposed vital points, that Thorfinn can get in the miniscule amount of time that the sun flash blinded Thorkell?
Heart is covered in chain mail, along with the rest of his vital organs. Taking out a tendon would work, but Thorkell would still have a working arm at worst, and then bye bye Thorfinn.
His neck is just as high as his chin, and in fact blocked by that chin.
Groin, probably would not be enough to incapacitate someone like him.