I used "truss head" screws to attached the decking. SS screws are a bit soft to drive into the aluminum stringers (stripped heads) so I tapped all holes with a steel screw, then inserted the SS screw into the tapped hole. Chop the head of a 3-4" long steel screw of the same size/pitch, chuck the screw into a drill, then use this as a tap to cut the threads into the aluminum stringers. Put a bit of wax on the screw so it doesn't bind in the aluminum.
I didn't use washers, but had a couple screws cut through the vinyl, so I put washers under those screws. I put a bit of wax under the screw head so it wouldn't grab the vinyl, which worked well except for a couple that I tightened down too much or hit a void...
You can also rivet the deck panels down, which is quicker, but if you ever need to remove a panel it's more difficult...
