How to do a Vang on an older Sunfish with a pulley?

I would like to rig a vang on an old Sunfish with a single deck pulley for the halyard. The pulley is a simple single pulley. I guess the only way to rig a vang would be to install a small double pulley like in the Old Sunfish upgrade thread I read where I got the photo below. Is there a way to do it that is class legal? Is the double pulley class legal?