KoshFan
Regular
2. The Vorlons were not wating for Valen when he arrived in the 1260s. He sent out a signal on a frequency he knew the Alliance used. The Vorlons reached him before the Minbari did, and were therefore with him when the Minbari boarded. The Vorlons of that time (the two who joined Valen on B4 were Kosh and Ulkesh) knew nothing about Valen.
Oh, to be a fly on the wall when Valen first spoke to Kosh!
Considering that Kosh and Ulkesh were, at that time, both wearing the same type of encounter suit, I wonder if Valen was still able to identify him. Or perhaps Valen simply said, going on a WAG, "Is one of you named Kosh?" Which would probably have made even the Vorlons sit up and take notice.
Once the Vorlons were convinced of Valen's authenticity, of course, then they no doubt thought it an excellent idea to unveil themselves to the first Minbari to come on board and lend Valen that much more authority.