Drill them out. Get a drill bit that is big enough to just take the head off of the rivet and then punch them out, that what I did to replace a set on my old WSM, piece of cake!!
+3 on the drill. Use a drill that is just a little smaller than the whole in the WSM. Once you clear out that much material, you can pretty much just pull the vent off. The metal isn't that strong. As for putting the new ones on, use stainless bolts and nuts. There is a locking nut that isn't nylon that you can snug up just right so that the vent works correctly and the nut will stay put. Any good hardware store should have them. Even though the link doesn't show it, they usually have a small dimple on two of the outer flats on the nut. Other options will work, but I like that set up the best.
http://www.fastenal.com/web/products/details/77798;jsessionid=FmcpJp1PQyHTFnhJKnmxzFh5pCZhzJhnMRX9Ngsnj2GJlJdxnpRC!1630393511!319881708?searchMode=productSearch&filterByVendingMachine=