Roman Legion in late Antiquity

[Frank]

With - a century being a standard 80+3 throughout (at full strength);

Which is as theroretical as your guess about the strength of the late roman legion. 8+)
Nice guess. Who is your 2nd and 3rd supernumerarius beside the centurio himself? And WHY?

But I like your idea, that the new legion was simply the usual vexillatio of 2 cohorts. Makes a lot of sense, in the tradition and the usual practice during 2nd and 3rd century. IIRC there are also hints, about 2 ranks of tribunes in late roman army. That would match with brigaded cohorts lead by the higher tribune, too.

However I doubt, that the legions of the limitanei in the 4th century were still about 3000 guys. They usually lost more than one vexillatio over the decades and they often never came back. So why should the romans refill old legions up to 8 cohorts accidentally? Makes no sense to me. I am afraid limitanei legions were much weaker.

But as with almost everything about late roman legions, we have no proof.