Yes, "συνερχομενην" is added by Vermigli. It agrees in case, number, and gender with "ἐκκλησίαν." So the form "συνερχομενην" is intentional, meaning "a gathering."
I think his point is that just as I Corinthians 16:19 shows inter-church greetings, with specific reference to "the church in their house" which is almost the exact same greek phrase in both passages, so also Romans 16:5 shows inter-church greetings. All of that is in contrast to a reference simply to the family of Prisca and Aquila.
καὶ τὴν κατ᾽ οἶκον αὐτῶν ἐκκλησίαν (Rom 16:5)
Ἀκύλας καὶ Πρίσκα σὺν τῇ κατ᾽ οἶκον αὐτῶν ἐκκλησίᾳ. (1Co 16:19)
19 The churches of Asia send you greetings. Aquila and Prisca, together with the church in their house, send you hearty greetings in the Lord. (1Co 16:19 ESV)
3 Greet Prisca and Aquila, my fellow workers in Christ Jesus, 4 who risked their necks for my life, to whom not only I give thanks but all the churches of the Gentiles give thanks as well. 5 Greet also the church in their house. (Rom 16:3-5 ESV)
Does that help?