Irule to disable VIP

The only way I can think of doing this is by checking the number of active members in the Pools used by the other Virtual Servers and if they are all down using the LB::down command to down all the Pool Members of the current VS which should bring the VS down. Of course if the VS shares an IP address with other VSs this isn't going to make a difference to RHI.

isn't using health monitor which checks another virtual server usable?

That's a great idea actually.

You could use a UDP monitor rather than ICMP