Commentary Par XXX 18

For the Latinism vice (here translated 'that which is due'), the commentators are torn among several possibilities.  Perhaps the most popular is the usage found in the Latin phrase explere vicem, meaning 'to fulfill one's duties,' probably the most likely sense of the word here.  But see Scartazzini (DDP Scartazzini.Par.XXX.18) for the majority opinion (which he does not share) that it means volta with the sense of 'time' or 'occasion.'  Several add 'place' to the possibilities, and there are still other options.  Singleton (DDP Singleton.Par.XXX.18) cites the other use of vice (at [Par XXVII 17]), where it is paired with officio (duty), to argue that it therefore cannot mean that here; but see Scartazzini (DDP Scartazzini.Par.XXVII.17-18) who deals with vice as there being the 'duty' of silence incumbent on the rest of the spirits while St. Peter fulfills his duty, which is to hold forth against papal turpitude, the two words sharing a sense approaching that of synonyms.

Scott (Scot.1977.2), p. 163, comments on the extraordinary incidence of Latinisms in this canto, which he puts at fifty.