I think Vermouth knew the Akai family in the past. During the mystery train case, Vermouth, as Scar Akai, said to Sera 'You haven't change at all, Masumi.' Not matte how well Vermouth can impersonate Akai, she would have no way of knowning how Sera was in the past and say with such confidents that she hasn't changed, if they have never met before, since I except that Akai keep his family and his job as FBI agent very much apart. Therefore I think that at some point in the past Vermouth have met Sera once.