I like duck cloth for its comfort and durability, and any kind of heavy twill is a good choice also. The vest I currently wear is cotton twill with a heavy lining, and it has survived many troops. And it's washable!
Pleather/vinyl is a very popular material for flak vests and it can look good, but a couple of caveats:
-It doesn't breathe, so if you are trooping in a warm climate, you will sweat. A lot. And pleather isn't as washable as fabrics which means you will need to hand wash it.
-There are many inexpensive vinyl materials out there, but most of them look really cheap. Unless you find something with a realistic finish, it is going to look fake. Marine vinyl is thick and durable, but also expensive and difficult to work with on the sewing machine.