I made my booking only one day before my trip. I was a bit worried if it is to late (as they require at least 24 hours in advance) so I called the number on the voucher after I made my booking. The lady was very nice and helpful and she confirmed it. She also advised that I am the only one who booked the tour but according to their rules they will do it even if it is one person who booked only. And yes, they did it for ONLY me and I had an amazing time. There was a very nice car came and pick me up from my hotel and there was a professional tour guide with the driver. Their English was perfect and they gave me a fantastic explanation about Vietnam, Hanoi and Halong Bay.
As soon as we arrived to Halon Bay and proceeded to the cruise I was advised that I am the only one who will be joining the boat. Lucky me :-) I saw same boats with 40 tourist doing the same cruising trip. I felt like a VIP to get that very special treatment.
I am absolutely satisfied with the service of the Travel Agency and all the Viator procedure. Definately I am going to book with them. I can only recommend for others to do the same thing. You dont have to worry about anything and you can reallz rely on them.