Eap'sDAD, thanks for checking back, glad you had no difficulty and you are having fun.
Hi Rose! I have not yet exchanged my voucher, but maybe I can help clarify. On our voucher only the lead travelers name appears and # of tickets. If I am to understand you correctly, you should be able to switch out a member of your party up until the point where vouchers are exchanged and tickets are redeemed at the turnstile, where as each ticket is THEN attached to a fingerprint. At that point, obviously, they are not transferable. That is my understanding, I hope it is correct.
PS I have had to call Viator about a few issues, and found CS to be very helpful, perhaps to be sure, you might give them a call.