I was not surprised that Vince was the other person.
Vince has sung with Don, Timothy and Joe and IMO has blended very well with them. Vince can blend well with anybody. Everybody in country music always wants Vince to sing or play on their record. Vince is one of the best guitarist in country music. Vince can sing low. So unless he's going to sing high along with Timothy(which he probably will), he might have to drop his range a little. They might have Vince sing Glenn's country rock songs and Deacon singing the other songs.
I don't know how the guitar leads will go because Vince do majority of his guitar solos in concert. He only do a few songs where he plays the acoustic guitar. I think Vince will fit very well. I think Vince will surprise us.
As for Steuart. Steuart will probably be there like always as part of the backup band. I would be very surprised if he wasn't, but I think he will be there. Its basically going to be 4 guitarist on stage(if Steuart will be there which he probably will). Also we never heard Steuart sing a lead before which is interesting. He just sing backup.
As for broadcasting the concert. I think Irving would be stupid to not do it.