Since I didn't get an answer here or from a direct Twitter message to IB, I called and it went o.k.: for the record, they first said that seat assignments in row 1 would have a charge, but not in rows 2 or 3, then he found that row 1 didn't have a charge; I'm not sure if it was because it was an award or because my wife asks for disability assistance.
He said that Vueling seats would need to be arranged separately; on the VY site, the booking couldn't be found, either in conjunction with my email or the airport codes, with the 6-character PNR the IB agent gave as the VY code, the 6-character IB code, or what's on the email confirming the seats: IB-slash-5 characters, with or without the IB/. I saw one thread here that somewhat indicates the problem, but not the solution. I'll ask here if there's an easy solution, and I see an online contact form with VY. I don't care all that much about seat assignments for the shorter flights, but it would be good not to find on the travel date that VY has no record of us.