Feeding the Soldiers

[D B Campbell]

... when he invents the 'ducentenarius' (leader of 200), a mistake for the 'ducenarius'.

I think Vegetius' mistake is in assuming that there was a unit of 200 men requiring a commander. From at least the time of Claudius, ducenarius indicated a procurator who was "paid 200" (i.e. 200 sesterces per annum).