iRule applied or not on virtual server.

There's two boxes in the iRules screen on the virtual, the left is "Enabled" -- these are actually configured on the virtual server. The right side is "Available" -- these are what are on the device and can be moved over to "Enabled."

You can reuse iRules on as many virtual servers as you want as long as they're written generically, such as an http to https redirect that uses the host header instead of hard coding the host into the redirect.