With that many reeves, 5/16" should be plenty for strength. Larger diameters might be easier on the hands. Dave's recommendation of XLS sounds good to me.
I find it useful to put sturdy snap hooks at both ends of the vang (unless you are rigging the boat for minimum weight for racing). That way, I can easily attach it other places & use the rig as a make shift crane for loading the boat, mounting an engine on the tender, stepping the mast, etc. I have also seen some people attach the bottom of the vang to a chain plate & use it as a preventer, when going down wind in puffy conditions.