Now that we've started on Viginius we might as well finish him off.
Ages ago, I said that Pseudo-Isidore likes to assign his letters discrete tasks: He'll often let one letter in the name of a given pope handle the historical hook, and another letter carry the meat. The Viginius letters are a textbook example of this recurring phenomenon. The first brings out all the key arguments; the second has the Liber Pontificalis tie-in (anemic though it may be).
According to his LP bio, Viginius was a Greek and an Athenian philosopher, so here we have Ps. Viginius popping off a letter to the Athenians. It consists almost enitrely of snippets from 2 Corinthians and 1 and 2 Peter, though Ps. Viginius adds the odd sentence of his own. In the opening lines he says that he's quite joyful and happy on the Athenians' behalf, because (his own words now) he's heard that they're doing things that befit good Christians. Sometimes I think our forgers could do with a good freshman writing course: Show, don't tell. That sort of thing.
The letter goes on to say a lot of stuff we've heard before. Mainly, the Athenians should avoid the company of the impious. Most of these exhortations come out of scripture, but Ps. Viginius does crop up about two-thirds of the way through to demand obedience to the apostolic see and emphasize that "great distance has to be kept between the faithful and the unfaithful." Then it's more scriptural citations to warn against impurity of the flesh (2 Cor. 7:1: "and the body!" Viginius adds, puzzlingly), and to advise that the day of the Lord will come like a thief.
Date: 20 February 138 (Magno et Camerino consulibus). Consuls as in the first Viginius letter, though the Liber Pontificalis offers consuls for both the beginning and the end of Viginius's pontificate, so the forgers could've changed it up if they'd wanted to.
Sources: the Bible (II Cor. 6:14, 16; 7:1, 4; I Peter 5:10-11; II Peter 2:4-10; 3:8-13); Liber Pontificalis (just for the historical hook and the consuls)