Good vinyl floor underlayment is the key to a quality finished floor. It has to be very smooth and stable. Unfortunately there's conflicting advice out there on what is the best vinyl underlayment.
The old standard for vinyl floor underlayment was lauan. Lauan underlayment is a cheap quarter-inch plywood. It's smooth but can have voids in the core and there are reports of delamination of the plys. That will ruin a flooring job real quick! It's also a rainforest product, so environmentalists don't like it.
I asked at my local lumberyard for a better vinyl floor underlayment and they recommended Accuply. It's a 5-ply underlayment that is 6mm thick. It certainly looks and feels sturdier than lauan. Accuply uses ultrasonic methods to check for voids and it's sanded very smooth. This vinyl underlayment also has the nailing pattern printed on the top. Sheets measure 4'x5'.
The earlier you install your vinyl floor underlayment, the better. That way you won't have to fit it around pipes, stub walls and other stuff. Installation is easy, just butt the sheets lightly together and nail at all the marked spots.