I personally wouldn't waste my money on the Mekong tour you have booked. You will be put in a very small boat that will go up and down the canals. This is NOT the Mekong-it is merely the small tributaries around the area. And you will have at least 5 hours of riding around in a bus/van. NOT a fun day.

For your arrival day, do the day tour of Saigon. Enjoy downtown Saigon at night. Shop til you drop. Maybe even go to the Sheraton or Caravelle and have dinner-the views are really great.

Then the following day, either take a REAL Mekong cruise such as Jade13 has booked or go to CuChi or get out in the country and see real Vietnamese life.

I think the first day tour you have planned leaves a LOT to be desired.