Nimbostratus
NacreousWill still work as long as you have a SelfIP in the target network segment (BigIP is then directly connected). As anywhere else in networking, Directly Connected takes precedence over static and dynamic routes. BigIP will refer to routes only if SelfIP is absent.
NacreousI did similar test more than a year ago. Hopefully I recall the details correctly.
Another VS will not be used as a backup. Virtual servers are strictly client-side and only one Virtual Server can match for one connection. If theres a default route in place, then that will be used as last resort. Obviously, default route is unlikely to get the request to right recipient, thus client is more than likely get a connection timeout error (connection drop). If there's no default route to refer to either, then BigIP will respond with TCP RST and client will get a "connection has been reset" error. Outcome for client will be exactly the same as if there were no pool members available for a standard type Virtual Server.